Optical Processing

ABSTRACT

A modular routing node includes a single input port and a plurality of output ports. The modular routing node is arranged to produce a plurality of different deflections and uses small adjustments to compensate for wavelength differences and alignment tolerances in an optical system. An optical device is arranged to receive a multiplex of many optical signals at different wavelengths, to separate the optical signals into at least two groups, and to process at least one of the groups adaptively.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/710,913, filed Feb. 23, 2010, which is a continuation of U.S.application Ser. No. 11/978,258, filed Oct. 29, 2007, now U.S. Pat. No.8,089,683, which is a continuation of U.S. application Ser. No.11/515,389, filed Sep. 1, 2006, now U.S. Pat. No. 7,612,930, which is adivisional of U.S. application Ser. No. 10/487,810, filed Sep. 2, 2002,now U.S. Pat. No. 7,145,710, which is the U.S. National Stage ofInternational Application No. PCT/GB02/04011, filed Sep. 2, 2002, andpublished in English. This application claims priority under 35 U.S.C.§119 or 365 to Great Britain Application No. 0121308.1, filed Sep. 3,2001. The entire teachings of the above applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical device and to a method ofcontrolling an optical device.

More particularly but not exclusively the invention relates to thegeneral field of controlling one or more light beams by the use ofelectronically controlled devices. The field of application is mainlyenvisaged as being to fields in which reconfiguration between inputs andoutputs is likely, and stability of performance is a significantrequirement.

BACKGROUND OF THE INVENTION

It has previously been proposed to use so-called spatial lightmodulators to control the routing of light beams within an opticalsystem, for instance from selected ones of a number of input opticalfibres to selected ones of output fibres.

Optical systems are subject to performance impairments resulting fromaberrations, phase distortions and component misalignment. An example isa multiway fibre connector, which although conceptually simple can oftenbe a critical source of system failure or insertion loss due to the verytight alignment tolerances for optical fibres, especially forsingle-mode optical fibres. Every time a fibre connector is connected,it may provide a different alignment error. Another example is anoptical switch in which aberrations, phase distortions and componentmisalignments result in poor optical coupling efficiency into theintended output optical fibres. This in turn may lead to high insertionloss. The aberrated propagating waves may diffract into intensityfluctuations creating significant unwanted coupling of light into otheroutput optical fibres, leading to levels of crosstalk that impedeoperation. In some cases, particularly where long path lengths areinvolved, the component misalignment may occur due to ageing ortemperature effects.

Some prior systems seek to meet such problems by use of expensivecomponents. For example in a communications context, known free-spacewavelength multiplexers and demultiplexers use expensive thermallystable opto-mechanics to cope with the problems associated with longpath lengths.

Certain optical systems have a requirement for reconfigurability. Suchreconfigurable systems include optical switches, add/drop multiplexersand other optical routing systems where the mapping of signals frominput ports to output ports is dynamic. In such systems thepath-dependent losses, aberrations and phase distortions encountered byoptical beams may vary from beam to beam according to the route taken bythe beam through the system. Therefore the path-dependent loss,aberrations and phase distortions may vary for each input beam or as afunction of the required output port.

The prior art does not adequately address this situation.

Other optical systems are static in terms of input/output configuration.In such systems, effects such as assembly errors, manufacturingtolerances in the optics and also changes in the system behaviour due totemperature and ageing, create the desirability for dynamic directioncontrol, aberration correction, phase distortion compensation ormisalignment compensation.

It should be noted that the features of dynamic direction control, phasedistortion compensation and misalignment control are not restricted tosystems using input beams coming from optical fibres. Such features mayalso be advantageous in a reconfigurable optical system. Another staticsystem in which dynamic control of phase distortion, direction and(relative) misalignment would be advantageous is one in which thequality and/or position of the input beams is time-varying.

Often the input and output beams for optical systems contain a multiplexof many optical signals at different wavelengths, and these signals mayneed to be separated and adaptively and individually processed insidethe system. Sometimes, although the net aim of a system is not toseparate optical signals according to their wavelength and then treatthem separately, to do so increases the wavelength range of the systemas a whole. Where this separation is effected, it is often advantageousfor the device used to route each channel to have a low insertion lossand to operate quickly.

It is an aim of some aspects of the present invention at least partly tomitigate difficulties of the prior art.

It is desirable for certain applications that a method or device foraddressing these issues should be polarisation-independent, or have lowpolarisation-dependence.

SLMs have been proposed for use as adaptive optical components in thefield of astronomical devices, for example as wavefront correctors. Inthis field of activity, the constraints are different to the presentfield-for example in communication and like devices, the need forconsistent performance is paramount if data is to be passed withouterrors. Communication and like devices are desirably inexpensive, anddesirably inhabit and successfully operate in environments that are notclosely controlled. By contrast, astronomical devices may be used inconditions more akin to laboratory conditions, and cost constraints areless pressing. Astronomical devices are unlikely to need to selectsuccessive routings of light within a system, and variations inperformance may be acceptable.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodof operating an optical device comprising an SLM having atwo-dimensional array of controllable phase-modulating elements, themethod comprising

delineating groups of individual phase-modulating elements;

selecting, from stored control data, control data for each group ofphase-modulating elements;

generating from the respective selected control data a respectivehologram at each group of phase-modulating elements; and

varying the delineation of the groups and/or the selection of controldata whereby upon illumination of said groups by respective light beams,respective emergent light beams from the groups are controllableindependently of each other.

In some embodiments, the variation of the delineation and/or controldata selection is in response to a signal or signals indicating anon-optimal performance of the device. In other embodiments, thevariation is performed during a set up or training phase of the device.In yet other embodiments, the variation is in response to an operatingsignal, for example a signal giving the result of sensingnon-performance system parameters such as temperature.

An advantage of the method of this aspect of the invention is thatstable operation can be achieved in the presence of effects such asageing, temperature, component, change of path through the system andassembly tolerances.

Preferably, control of said light beams is selected from the groupcomprising: control of direction, control of power, focussing,aberration compensation, sampling and beam shaping.

Clearly in most situations more than one of these control types will beneeded-for example in a routing device (such as a switch, filter oradd/drop multiplexer) primary changes of direction are likely to beneeded to cope with changes of routing as part of the main system butsecondary correction will be needed to cope with effects such astemperature and ageing. Additionally, such systems may also need tocontrol power, and to allow sampling (both of which may in some cases beachieved by direction changes).

Advantageously, each phase modulating element is responsive to arespective applied voltage to provide a corresponding phase shift toemergent light, and the method further comprises;

controlling said phase-modulating elements of the spatial lightmodulator to provide respective actual holograms derived from therespective generated holograms, wherein the controlling step comprises;

resolving the respective generated holograms modulo 2pi.

The preferred SLM uses a liquid crystal material to provide phase shiftand the liquid crystal material is not capable of large phase shiftsbeyond plus or minus 2π. Some liquid crystal materials can only providea smaller range of phase shifts, and if such materials are used, theresolution of the generated hologram is correspondingly smaller.

Preferably the method comprises:

providing a discrete number of voltages available for application toeach phase modulating element;

on the basis of the respective generated holograms, determining thedesired level of phase modulation at a predetermined point on each phasemodulating element and choosing for each phase modulating element theavailable voltage which corresponds most closely to the desired level.

Where a digital control device is used, the resolution of the digitalsignal does not provide a continuous spectrum of available voltages. Oneway of coping with this is to determine the desired modulation for eachpixel and to choose the individual voltage which will provide theclosest modulation to the desired level.

In another embodiment, the method comprises:

providing a discrete number of voltages available for application toeach phase modulating element;

determining a subset of the available voltages which provides the bestfit to the generated hologram.

Another technique is to look at the pixels of the group as a whole andto select from the available voltages those that give rise to thenearest phase modulation across the whole group.

Advantageously, the method further comprises the step of storing saidcontrol data wherein the step of storing said control data comprisescalculating an initial hologram using a desired direction change of abeam of light, applying said initial hologram to a group of phasemodulating elements, and correcting the initial hologram to obtain animproved result.

The method may further comprise the step of providing sensors fordetecting temperature change, and performing said varying step inresponse to the outputs of those sensors.

The SLM may be integrated on a substrate and have an integralquarter-wave plate whereby it is substantially polarisation insensitive.

Preferably the phase-modulating elements are substantially reflective,whereby emergent beams are deflected from the specular reflectiondirection.

In some aspects, for at least one said group of pixels, the methodcomprises providing control data indicative of two holograms to bedisplayed by said group and generating a combined hologram before saidresolving step.

According to a second aspect of the invention, there is provided anoptical device comprising an SLM and a control circuit, the SLM having atwo-dimensional array of controllable phase-modulating elements and thecontrol circuit having a store constructed and arranged to hold pluralitems of control data, the control circuit being constructed andarranged to delineate groups of individual phase-modulating elements, toselect, from stored control data, control data for each group ofphase-modulating elements, and to generate from the respective selectedcontrol data a respective hologram at each group of phase-modulatingelements,

wherein the control circuit is further constructed and arranged, to varythe delineation of the groups and/or the selection of control data

whereby upon illumination of said groups by respective light beams,respective emergent light beams from the groups are controllableindependently of each other.

An advantage of the device of this aspect of the invention is thatstable operation can be achieved in the presence of effects such asageing, temperature, component and assembly tolerances. Embodiments ofthe device can handle many light beams simultaneously. Embodiments canbe wholly reconfigurable, for example compensating differently for anumber of routing configurations.

Preferably, the optical device has sensor devices arranged to detectlight emergent from the SLM, the control circuit being responsive tosignals from the sensors to vary said delineation and/or said selection.

In some embodiments, the optical device has temperature responsivedevices constructed and arranged to feed signals indicative of devicetemperature to said control circuit, whereby said delineation and/orselection is varied.

In another aspect, the invention provides an optical routing devicehaving at least first and second SLMs and a control circuit, the firstSLM being disposed to receive respective light beams from an input fibrearray, and the second SLM being disposed to receive emergent light fromthe first SLM and to provide light to an output fibre array, the firstand second SLMs each having a respective two-dimensional array ofcontrollable phase-modulating elements and the control circuit having astore constructed and arranged to hold plural items of control data, thecontrol circuit being constructed and arranged to delineate groups ofindividual phase-modulating elements, to select, from stored controldata, control data for each group of phase-modulating elements, and togenerate from the respective selected control data a respective hologramat each group of phase-modulating elements,

wherein the control circuit is further constructed and arranged, to varythe delineation of the groups and/or the selection of control data

whereby upon illumination of said groups by respective light beams,respective emergent light beams from the groups are controllableindependently of each other.

In a further aspect, the invention provides a device for shaping one ormore light beams in which the or each light beam is incident upon arespective group of pixels of a two-dimensional SLM, and the pixels ofthe or each respective group are controlled so that the correspondingbeams emerging from the SLM are shaped as required.

According to a further aspect of the invention there is provided anoptical device comprising one or more optical inputs at respectivelocations, a diffraction grating constructed and arranged to receivelight from the or each optical input, a focussing device and acontinuous array of phase modulating elements, the diffraction gratingand the array of phase modulating elements being disposed in the focalplane of the focussing device whereby diverging light from a singlepoint on the diffraction grating passes via the focussing device to formbeams at the array of phase modulating elements, the device furthercomprising one or more optical output at respective locations spatiallyseparate from the or each optical input, whereby the diffraction gratingis constructed and arranged to output light to the or each opticaloutput.

This device allows multiwavelength input light to be distributed inwavelength terms across different groups of phase-modulating elements.This allows different processing effects to be applied to any desiredpart or parts of the spectrum.

According to a still further aspect of the invention there is provided amethod of filtering light comprising applying a beam of said light to adiffraction grating whereby emerging light from the grating is angularlydispersed by wavelength, forming respective beams from said emerginglight by passing the emerging light to a focussing device having thegrating at its focal plane, passing the respective beams to an SLM atthe focal plane of the focussing device, the SLM having atwo-dimensional array of controllable phase-modulating elements,selectively reflecting light from different locations of said SLM andpassing said reflected light to said focussing element and then to saidgrating.

Preferably the method comprises delineating groups of individualphase-modulating elements to receive beams of light of differingwavelength;

selecting, from stored control data, control data for each group ofphase-modulating elements;

generating from the respective selected control data a respectivehologram at each group of phase-modulating elements; and

varying the delineation of the groups and/or the selection of controldata.

According to a still further aspect of the invention there is providedan optical add/drop multiplexer having a reflective SLM having atwo-dimensional array of controllable phase-modulating elements, adiffraction device and a focussing device wherein light beams from acommon point on the diffraction device are mutually parallel whenincident upon the SLM, and wherein the SLM displays respective hologramsat locations of incidence of light to provide emergent beams whosedirection deviates from the direction of specular reflection.

In a yet further aspect, the invention provides a test or monitoringdevice comprising an SLM having a two-dimensional array of pixels, andoperable to cause incident light to emerge in a direction deviating fromthe specular direction, the device having light sensors at predeterminedlocations arranged to provide signals indicative of said emerging light.

The test or monitoring device may further comprise further sensorsarranged to provide signals indicative of light emerging in the speculardirections.

Yet a further aspect of the invention relates to a power control devicefor one or more beams of lights in which the said beams are incident onrespective groups of pixels of a two-dimensional SLM, and holograms areapplied to the respective group so that the emergent beams have powerreduced by comparison to the respective incident beams.

The invention further relates to an optical routing module having atleast one input and at least two outputs and operable to select betweenthe outputs, the module comprising a two dimensional SLM having an arrayof pixels, with circuitry constructed and arranged to display hologramson the pixels to route beams of different frequency to respectiveoutputs.

According to a later aspect of the invention there is provided anoptoelectronic device comprising an integrated multiple phase spatiallight modulator (SLM) having a plurality of pixels, wherein each pixelcan phase modulate light by a phase shift having an upper and a lowerlimit, and wherein each pixel has an input and is responsive to a valueat said input to provide a phase modulation determined by said value,and a controller for the SLM, wherein the controller has a control inputreceiving data indicative of a desired phase modulation characteristicacross an array of said pixels for achieving a desired control of lightincident on said array, the controller has outputs to each pixel, eachoutput being capable of assuming only a discrete number of possiblevalues, and the controller comprises a processor constructed andarranged to derive, from said desired phase modulation characteristic, anon-monotonic phase modulation not extending outside said upper andlower limits, and a switch constructed and arranged to select betweenthe possible values to provide a respective one value at each outputwhereby the SLM provides said non-monotonic phase modulation.

Some or all of the circuitry may be on-chip leading to built-inintelligence. This leads to more compact and ultimately low-costdevices. In some embodiments, some or all on-chip circuitry may operatein parallel for each pixel which may provide huge time advantages; inany event the avoidance of the need to transfer data off chip andthereafter to read in to a computer allows configuration andreconfiguration to be faster.

According to another aspect of the invention there is provided a methodof controlling a light beam using a spatial light modulator (SLM) havingan array of pixels, the method comprising:

determining a desired phase modulation characteristic across a sub-arrayof said pixels for achieving the desired control of said beam;

controlling said pixels to provide a phase modulation derived from thedesired phase modulation, wherein the controlling step comprises

providing a population of available phase modulation levels for eachpixel, said population comprising a discrete number of said phasemodulation levels;

on the basis of the desired phase modulation, a level selecting step ofselecting for each pixel a respective one of said phase modulationlevels; and

causing each said pixel to provide the respective one of said phasemodulation levels.

The SLM may be a multiple phase liquid crystal over silicon spatiallight modulator having plural pixels, of a type having an integratedwave plate and a reflective element, such that successive passes of abeam through the liquid crystal subject each orthogonally polarisedcomponent to a substantially similar electrically-set phase change.

If a non-integrated wave plate is used instead, a beam after reflectionand passage through the external wave plate will not pass through thesame zone of the SLM, unless it is following the input path, in whichcase the zero order component of said beam will re-enter the inputfibre.

The use of the wave plate and the successive pass architecture allowsthe SLM to be substantially polarisation independent.

In one embodiment the desired phase modulation at least includes alinear component.

Linear phase modulation, or an approximation to linear phase modulationmay be used to route a beam of light, i.e. to select a new direction ofpropagation for the beam. In many routing applications, two SLMs areused in series, and the displayed information on the one has the inverseeffect to the information displayed on the other. Since the informationrepresents phase change data, it may be regarded as a hologram. Hence anoutput SLM may display a hologram that is the inverse of that displayedon the input SLM. Routing may also be “one-to-many” (i.e. multicasting)or “one-to-all” (i.e. broadcasting) rather than the more usualone-to-one in many routing devices. This may be achieved by correctselection of the relevant holograms.

Preferably the linear modulation is resolved modulo 2pi to provide aperiodic ramp.

In another embodiment the desired phase modulation includes a non-linearcomponent.

Preferably the method further comprises selecting, from said array ofpixels, a sub-array of pixels for incidence by said light beam.

The size of a selected sub-array may vary from switch to switchaccording to the physical size of the switch and of the pixels. However,a typical routing device may have pixel arrays of between 100*100 and200*200, and other devices such as add/drop multiplexers may have arraysof between 10*10 and 50*50. Square arrays are not essential.

In one embodiment the level-selecting step comprises determining thedesired level of phase modulation at a predetermined point on each pixeland choosing for each pixel, the available level which corresponds mostclosely to the desired level.

In another embodiment, the level-selecting step comprises determining asubset of the available levels, which provides the best fit to thedesired characteristic.

The subset may comprise a subset of possible levels for each pixel.

Alternatively the subset may comprise a set of level distributions, eachhaving a particular level for each pixel.

In one embodiment, the causing step includes providing a respectivevoltage to an electrode of each pixel, wherein said electrode extendsacross substantially the whole of the pixel.

Preferably again the level selecting step comprises selecting the levelby a modulo 2pi comparison with the desired phase modulation. The actualphase excursion may be from A to A+2π where A is an arbitrary angle.

Preferably the step of determining the desired phase modulationcomprises calculating a direction change of a beam of light.

Conveniently, after the step of calculating a direction change, the stepof determining the desired phase modulation further comprises correctingthe phase modulation obtained from the calculating step to obtain animproved result.

Advantageously, the correction step is retroactive.

In another embodiment the step of determining the desired phasemodulation is retroactive, whereby parameters of the phase modulationare varied in response to a sensed error to reduce the error.

A first class of embodiments relates to the simulation/synthesis ofgenerally corrective elements. In some members of the first class, themethod of the invention is performed to provide a device, referred tohereinafter as an accommodation element for altering the focus of thelight beam.

An example of an accommodation element is a lens. An accommodationelement may also be an anti-astigmatic device, for instance comprisingthe superposition of two cylindrical lenses at arbitrary orientations.

In other members of the first class, the method of the invention isperformed to provide an aberration correction device for correctinggreater than quadratic aberrations.

The sub-array selecting step may assign a sub-array of pixels to a beambased on the predicted path of the beam as it approaches the SLM justprior to incidence.

Advantageously, after the sub-array is assigned using the predictedpath, it is determined whether the assignment is correct, and if not adifferent sub-array is assigned.

The assignment may need to be varied in the event of temperature, ageingor other physical changes. The sub-array selection is limited inresolution only by the pixel size. By contrast other array devices suchas MEMS have fixed physical edges to their beam steering elements.

An element of this type may be used in a routing device to compensatefor aberrations, phase distortions and component misalignment in thesystem. By providing sensing devices a controller may be used toretroactively control the element and the element may maintain anoptimum performance of the system.

In one embodiment of this first class, the method includes both causingthe SLM to route a beam and causing the SLM to emulate a correctiveelement to correct for errors, whereby the SLM receives a discreteapproximation of the combination of both a linear phase modulationapplied to it to route the beam and a non-linear phase modulation forsaid corrections.

Synthesising a lens using an SLM can be used to change the position ofthe beam focused spot and therefore correct for a position error ormanufacturing tolerance in one or more other lenses or reflective (asopposed to transmissive) optical elements such as a curved mirror.

The method of the invention may be used to correct for aberrations suchas field curvature in which the output ‘plane’ of the image(s) from anoptical system is curved, rather than flat.

In another embodiment of the first class, intelligence may be integratedwith sensors that detect the temperature changes and apply data from alook-up table to apply corrections.

In yet another embodiment of this class, misalignment and focus errorsare detected by measuring the power coupled into strategically placedsensing devices, such as photodiode arrays, monitor fibres or awavefront sensor. Compensating holograms are formed as a result of thediscrete approximations of the non-linear modulation. Changes oradjustments may then be made to these holograms, for example by applyinga stimulus and then correcting the holograms according to the sensedresponse until the system alignment is measured to be optimised.

In embodiments where the method provides routing functions byapproximated linear modulation, adaptation of non-linear modulation dueto changes in the path taken through the system desirably takes place ona timescale equivalent to that required to change the hologram routing,i.e. of the order of milliseconds.

A control algorithm may use one or more of several types ofcompensation.

In one embodiment a look-up table is used with pre-calculated ‘expected’values of the compensation taking account of the different routesthrough the system.

In another embodiment the system is trained before first being operated,by repeated changes of, or adjustments to, the compensating holograms tolearn how the system is misaligned.

A further embodiment employs intelligence attached to the monitor fibresfor monitoring and calculation of how these compensating hologramsshould adapt with time to accommodate changes in the system alignment.This is achieved in some embodiments by integrating circuitry componentsinto the silicon backplane of the SLM.

In many optical systems there is a need to control and adapt the poweror shape of an optical beam as well as its direction or route throughthe optical system. In communications applications, power control isrequired for network management reasons. In general, optical systemsrequire the levelling out or compensation for path andwavelength-dependent losses inside the optical system. It is usuallydesirable that power control should not introduce or accentuate otherperformance impairments.

Thus in a second class of embodiments, the modulation applied ismodified for controlling the attenuation of an optical channel subjectedto the SLM.

In one particular embodiment, the ideal value of phase modulation iscalculated for every pixel, and then multiplied by a coefficient havinga value between 0 and 1, selected according to the desired attenuationand the result is compared to the closest available phase level toprovide the value applied to the pixels.

In another embodiment, the method further comprises selecting by adiscrete approximation to a linear phase modulation, a routing hologramfor display by the SLM whereby the beams may be correctly routed;selecting by a discrete approximation to a non-linear phase modulation,a further hologram for separating each beam into main and subsidiarybeams, wherein the main beam is routed through the system and the oreach subsidiary beam is diffracted out of the system; combining therouting and further holograms together to provide a resultant hologram;and causing the SLM to provide the resultant hologram.

The non-linear phase modulation may be oscillatory.

In yet another embodiment, the method further comprises selecting by adiscrete approximation to a linear phase modulation, a routing hologramfor display by the SLM whereby the beams may be correctly routed;selecting by a discrete approximation to a non-linear phase modulation,a further hologram for separating each beam into main and subsidiarybeams, wherein the main beam is routed through the system and at leastone subsidiary beam is incident on an output at an angle such that itscontribution is insignificant; combining the routing and furtherholograms together to provide resultant hologram; and causing the SLM todisplay the resultant hologram.

The non-linear phase modulation may be oscillatory.

In a closely allied class of embodiments, light may be selectivelyrouted to a sensor device for monitoring the light in the system. Thetechnique used may be a power control technique in which light divertedfrom the beam transmitted through the system to reduce its magnitude ismade incident on the sensor device.

In another class of embodiments, a non-linear phase modulation profileis selected to provide beam shaping, for example so as to reducecross-talk effects due to width clipping. This may use a pseudoamplitude modulation technique.

In a further class of embodiments, the method uses a non-linearmodulation profile chosen to provide wavelength dependent effects.

The light may be at a telecommunications wavelength, for example 850 nm,1300 nm or in the range 1530 nm to 1620 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 shows a cross-sectional view through an exemplary SLM suitablefor use in the invention;

FIG. 2 shows a sketch of a routing device in which a routing SLM is usedadditionally to provide correction for performance impairment due tomisalignment;

FIG. 3 shows a sketch of a routing device in which a routing SLM is usedto route light beams and an additional SLM provides correction forperformance impairment due to misalignment;

FIG. 4 shows a block diagram of an adaptive corrective SLM;

FIG. 5 shows an adaptive optical system using three SLMs;

FIG. 6 shows a partial block diagram of a routing device with a dualfunction SLM and control arrangements;

FIG. 7 shows a block diagram of an SLM for controlling the powertransferred in an optical system;

FIG. 8 a shows a diagram of phase change distribution applied by ahologram for minimum attenuation;

FIG. 8 b shows a diagram of phase change distribution applied by ahologram enabling attenuation of the signal;

FIG. 9 shows a power control system;

FIG. 10 shows a phasor diagram showing the effect of non-linearoscillatory phase modulation applied to adjacent pixels;

FIG. 11 shows a schematic diagram of a part of an optical routing systemillustrating the effects of clipping and cross talk;

FIG. 12 shows a partial block diagram of a system enabling beams ofdifferent wavelength from a composite input beam to be separatelycontrolled before recombination; and

FIGS. 13 a and 13 b show schematic diagrams of an add/drop multiplexerusing an SLM assuming 1-D routing.

FIG. 14 is a diagram similar to FIG. 12 but showing a magnificationstage for increasing the effective beam deflection angle;

FIG. 15 shows a vector diagram of the operation of an add/dropmultiplexer;

FIG. 16 shows a block diagram showing how loop back may be effected;

FIG. 17 is a vector diagram illustrating the operation of part of FIG.16;

FIG. 18 is a vector diagram of a multi-input/multi-output architecture;

FIG. 19 is a graph showing the relative transmission Tlo for in-bandwavelengths as a function of the ratio of the wavelength offset u tocentre of the wavelength channel separation;

FIG. 20 is a graph showing the relative transmission Thi inside adjacentchannels;

FIG. 21 shows a logical diagram of the sorting function;

FIG. 22 shows a block diagram of an add/drop node using two routingmodules;

FIG. 23 shows a block diagram of modules used to cross-connect tworings;

FIG. 24 shows a block diagram of routing modules connected to provideexpansion;

FIG. 25 shows a block diagram of an optical cross-connect;

FIG. 26 shows a block diagram of an upgrades node having a cascadedmodule at an expansion output port;

FIG. 27 is a graph showing the effect of finite hologram size of thefield of a beam incident on a hologram;

FIG. 28 shows a schematic layout of a wavelength filter device; and,

FIG. 29 shows a schematic layout of an add/drop device;

FIG. 30 shows a block diagram of an optical test set;

FIG. 31 is a diagram showing the effect of finite hologram size on abeam at a wavelength different to the centre wavelength associated withthe hologram;

FIG. 32 shows the truncated beam shapes for wavelengths at variouswavelength differences from the centre of the wavelength channel droppedin isolation;

FIG. 33 shows the overlap integrands of the beams of FIG. 32 with thefundamental mode of the fibre;

FIG. 34 shows beam output positions for different wavelengths withrespect to two optical fibres; and

FIG. 35 shows the overlap integrand between the beams of FIG. 34 and thefundamental mode of one of the optical fibres.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Many of the embodiments of the invention centre upon the realisationthat the problems of the prior art can be solved by using a reflectiveSLM having a two-dimensional array of phase-modulating elements that islarge in number, and applying a number of light beams to groups of thosephase-modulating elements. A significant feature of these embodiments isthe fact that the size, shape and position of those groups need not befixed and can, if need be, be varied. The groups may display hologramswhich can be set up as required to deflect the light so as to provide anon-specular reflection at a controllable angle to the specularreflection direction. The holograms may additionally or alternativelyprovide shaping of the beam.

The SLM may thus simulate a set of highly flexible mirrors, one for eachbeam of light. The size, shape and position of each mirror can bechanged, as can the deflection and the simulated degree of curvature.

Devices embodying the invention act on light beams incident on thedevice to provide emerging light beams which are controlledindependently of one another. Possible types of control include controlof direction, control of power, focussing, aberration compensation,sampling and beam shaping.

The structure and arrangement of polarisation-independent multiple phaseliquid crystal over silicon spatial light modulators (SLMs) for routinglight beams using holograms are discussed in our co-pending patentapplication PCT/GB00/03796. Such devices have an insertion loss penaltydue to the dead-space between the pixels. As discussed in our co-pendingpatent application GB0107742.9, the insertion loss may be reducedsignificantly by using a reflecting layer inside the substratepositioned so as to reflect the light passing between the pixels backout again.

Referring to FIG. 1, an integrated SLM 200 for modulating light 201 of aselected wavelength, e.g. 1.5 μm, consists of a pixel electrode array230 formed of reflective aluminum. The pixel electrode array 230, aswill later be described acts as a mirror, and disposed on it is aquarter-wave plate 221. A liquid crystal layer 222 is disposed on thequarter-wave plate 221 via an alignment layer (not shown) as is known tothose skilled in the art of liquid crystal structures. Over (as shown)the liquid crystal layer 222 are disposed in order a second alignmentlayer 223, a common ITO electrode layer 224 and an upper glass layer225. The common electrode layer 224 defines an electrode plane. Thepixel electrode array 230 is disposed parallel to the common electrodeplane 224. It will be understood that alignment layers and otherintermediate layers will be provided as usual. They are omitted in FIG.1 for clarity.

The liquid crystal layer 222 has its material aligned such that underthe action of a varying voltage between a pixel electrode 230 and thecommon electrode 224, the uniaxial axis changes its tilt direction in aplane normal to the electrode plane 224.

The quarter wave plate 221 is disposed such that light polarised in theplane of tilt of the director is reflected back by the mirror 230through the SLM with its plane of polarisation perpendicular to theplane of tilt, and vice-versa.

Circuitry, not shown, connects to the pixel electrodes 230 so thatdifferent selected voltages are applied between respective pixelelectrodes 230 and the common electrode layer 224.

Considering an arbitrary light beam 201 passing through a given pixel,to which a determined potential difference is applied, thus resulting ina selected phase modulation due to the liquid crystal layer over thepixel electrode 230. Consider first and second orthogonal polarisationcomponents, of arbitrary amplitudes, having directions in the plane oftilt of the director and perpendicular to this plane, respectively.These directions bisect the angles between the fast and slow axes of thequarter-wave plate 221.

The first component experiences the selected phase change on the inwardpass of the beam towards the aluminium layer 230, which acts as amirror. The second component experiences a fixed, non-voltage dependentphase change

However, the quarter-wave plate 221 in the path causes polarisationrotation of the first and second components by 90 degrees so that thesecond polarisation component of the light beam is presented to theliquid crystal for being subjected to the selected phase change on theoutward pass of the beam away from the mirror layer 230. The firstpolarisation component experiences the fixed, non-voltage dependentphase change on the outward pass of the beam. Thus, both of thecomponents experience the same overall phase change contribution afterone complete pass through the device, the total contribution being thesum of the fixed, non-voltage dependent phase and the selected voltagedependent phase change.

It is not intended that any particular SLM structure is essential to theinvention, the above being only exemplary and illustrative. Theinvention may be applied to other devices, provided they are capable ofmultiphase operation and are at least somewhat polarisation independentat the wavelengths of concern. Other SLMs are to be found in ourco-pending applications WO01/25840, EP1050775 and EP1053501 as well aselsewhere in the art.

Where liquid crystal materials other than ferroelectric are used,current practice indicates that the use of an integral quarter waveplate contributes to the usability of multiphase,polarisation-independent SLMs.

A particularly advantageous SLM uses a liquid crystal layer configuredas a pi cell.

Referring to FIG. 2, an integrated SLM 10 has processing circuitry 11having a first control input 12 for routing first and second beams 1,2from input fibres 3,4 to output fibres 5,6 in a routing device 15. Theprocessing circuitry 11 includes a store holding control data which isprocessed to generate holograms which are applied to the SLM 10 forcontrol of light incident upon the SLM 10. The control data are selectedin dependence upon the data at the control input 12, and may be storedin a number of ways, including compressed formats. The processingcircuitry 11, which may be at least in part on-chip, is also shown ashaving an additional input 16 for modifying the holograms. This input 16may be a physical input, or may be a “soft” input-for example data in aparticular time slot.

The first beam 1 is incident on, and processed by a first array, orblock 13 of pixels, and the second beam 2 is incident on and processedby a second array, or block 14 of pixels. The two blocks of pixels 13,14are shown as contiguous. In some embodiments they might however beseparated from one another by pixels that allow for misalignment.

Where the SLM is used for routing the beams 1,2 of light, this isachieved by displaying a linearly changing phase ramp in at least onedirection across the blocks or arrays 13,14. The processing circuitry 11determines the parameters of the ramp depending on the required angle ofdeflection of the beam 1,2. Typically the processing circuitry 11 storesdata in a look-up table, or has access to a store of such data, toenable the required ramp to be created in response to the input data orcommand at the first control input 12. The angle of deflection isprobably a two dimensional angle where the plane common to the directionof the incident light and that of the reflected light is not orthogonalto the SLM.

Assigning x and y co-ordinates to the elements of the SLM, the requiredamount of angular shift from the specular reflection direction may beresolved into the x and y directions. Then, the required phase ramp forthe components is calculated using standard diffraction theory, as a“desired phase characteristic”.

This process is typically carried out in a training stage, to providethe stored data in the look-up table.

Having established a desired phase modulation characteristic across thearray so as to achieve the desired control of said beam the processingcircuitry 11 transforms this characteristic into one that can bedisplayed by the pixels 13,14 of the SLM 10. Firstly it should be bornein mind that the processing circuitry 11 controlling the pixels of anSLM 10 is normally digital. Thus there is only a discrete population ofvalues of phase modulation for each pixel, depending on the number ofbits used to represent those states.

To allow the pixels 13,14 of the SLM 11 to display a suitable phaseprofile, the processing circuitry 11 carries out a level selectingoperation for each pixel. As will be appreciated, the ability of the SLMto phase modulate has limits due to the liquid crystal material, andhence a phase ramp that extends beyond these limits is not possible. Toallow for the physical device to provide the effects of the ideal device(having a continuously variable limitless phase modulation ability), thedesired phase ramp may be transformed into a non-monotonic variationhaving maxima and minima within the capability limits of the SLM 10. Inone example of this operation, the desired phase modulation is expressedmodulo 2pi across the array extent, and the value of the desiredmodulo-2pi modulation is established at the centre of each pixel. Thenfor each pixel, the available level nearest the desired modulation isascertained and used to provide the actual pixel voltage. This voltageis applied to the pixel electrode for the pixel of concern.

For small pixels there may be edge effects due to fringing fieldsbetween the pixels and the correlations between the director directionsin adjacent pixels. In such systems the available phase level nearest tothe value of the desired modulo-2pi modulation at the centre of eachpixel (as described above) should be used as a first approximation. Arecursive algorithm is used to calculate the relevant system performancecharacteristic taking into account these ‘edge’ effects and to changethe applied level in order to improve the system performance to therequired level.

“Linear” means that the value of phase across an array of pixels varieslinearly with distance from an arbitrary origin, and includes limitedlinear changes, where upon reaching a maximum phase change at the end ofa linear portion, the phase change reverts to a minimum value beforeagain rising linearly.

The additional input 16 causes the processing circuitry 11 to modify theholograms displayed by applying a discrete approximation of a non-linearphase modulation so that the SLM 10 synthesises a corrective opticalelement such as a lens or an aberration corrector. As will be laterdescribed, embodiments may also provide power control (attenuation),sampling and beam shaping by use of the non-linear phase modulationprofile. “Non-linear” is intended to signify that the desired phaseprofile across an array of pixels varies with distance from an arbitraryorigin in a curved and/or oscillatory or like manner that is not alinear function of distance. It is not intended that “non-linear” referto sawtooth or like profiles formed by a succession of linear segmentsof the same slope mutually separated by “flyback” segments.

The hologram pattern associated with any general non-linear phasemodulation exp jφ(u)=exp j(φ₀(u)+φ₁(u)+φ₃(u) . . . ) where j is thecomplex operator, can be considered as a product. In this product, thefirst hologram term in the product exp j φ₀(u) implements the routingwhile the second hologram term exp j φ₁(u) implements a correctivefunction providing for example lens simulation and/or aberrationcorrection. The third hologram term exp j φ₂(u) implements a signalprocessing function such as sampling and/or attenuation and/or beamshaping. The routing function is implemented as a linear phasemodulation while the corrective function includes non-linear terms andthe signal processing function includes non-linear oscillatory terms.

Different methods of implementing the combination of these three termsare possible. In one embodiment the total required phase modulationφ₀(u)+φ₁(u)+φ₂(u) including linear routing and corrective function andthe signal processing function is resolved modulo 2pi and approximatedto the nearest available phase level before application by the pixels.In another embodiment the summation of the phase modulation required forthe linear and corrective function φ₀(u)+φ₁(u) is resolved modulo 2piand approximated to the nearest phase level in order to calculate afirst phase distribution. A second phase distribution φ₂(u) iscalculated to provide sampling and/or attenuation and/or beam shaping.The two phase distributions are then added, re-resolved modulo 2pi andapproximated to the nearest available phase level before application bythe pixels. Other methods are also possible.

Mathematically the routing phase modulation is periodic due to theresolution modulo 2pi and by nature of its linearity.

Therefore the routing phase modulation results in a set of equallyspaced diffraction orders. The greater the number of available phaselevels the closer the actual phase modulation to the ideal value and thestronger the selected diffraction order used for routing.

By contrast, the corrective effects are realised by non-linear phasechanges φ₁(u) that are therefore non-periodic when resolved modulo 2pi.This non-periodic phase modulation changes the distribution of thereflected beam about its centre, but not its direction. The combinedeffect of both linear (routing) and non-periodic phase modulation is tochange both the direction and distribution of the beam, as may be shownusing the convolution theorem.

The signal processing effects are usually realised by a methodequivalent to ‘multiplying’ the initial routing and/or hologram expj(φ₀(u)+φ₁(u)) by a further hologram exp j φ₂(u) in which φ₂(u) isnon-linear and oscillatory. Therefore the set of diffraction ordersassociated with the further hologram creates a richer structure ofsubsidiary beams about the original routed beam, as may be shown usingthe convolution theorem.

While this explanation is for a one-dimensional phase modulator arraythe same principle may be applied in 2-D.

Hence in a reconfigurable optical system this non-linear phasemodulation may be applied by the same spatial light modulator(s) thatroute the beam. It will be understood by those skilled in the art thatthe SLM may have only a single control input and the device may haveprocessing circuitry for combining control data for routing and controldata for corrective effects and signal processing effects to provide anoutput to control the SLM.

The data may be entered into the SLM bit-wise per pixel so that for eachpixel a binary representation of the desired state is applied.Alternatively, the data may be entered in the form of coefficients of apolynomial selected to represent the phase modulation distribution ofthe pixel array of concern in the SLM. This requires calculating abilityof circuitry of the SLM, but reduces the data transfer rates into theSLM. In an intermediate design the polynomial coefficients are receivedby a control board that itself sends bit-wise per pixel data to the SLM.On-chip circuitry may interpret data being entered so as to decompressthat data.

The pixel array of concern could be all of the pixels associated with aparticular beam or a subset of these pixels. The phase modulationdistribution could be a combined phase modulation distribution for bothrouting and corrective effects or separate phase modulationdistributions for each. Beam shaping, sampling and attenuation phasemodulation distributions, as will be described later, can also beincluded. In some cases it may not be possible to represent the phasemodulation distribution as a simple polynomial. This difficulty may beovercome by finding a simple polynomial giving a first approximation tothe desired phase modulation distribution. The coefficients of thispolynomial are sent to the SLM. A bit-wise correction is sent for eachpixel requiring a correction, together with an address identifying thelocation of the pixel. When the applied distribution is periodic onlythe corrections for one period need be sent.

The processing circuitry 11 may be discrete from or integral with theSLM, or partly discrete and partly integral.

Referring to FIG. 3, a routing device 25 includes two SLMs 20,21 whichdisplay holograms for routing light 1,2 from an input fibre array 3,4 toan output fibre array 5,6. The two SLMs are reflective and define azigzag path. The first SLM 20 hereinafter referred to as a “correctiveSLM” not only carries out routing but also synthesises a correctiveoptical element. The second SLM 21 carries out only a routing functionin this embodiment, although it could also carry out corrections orapply other effects if required. The second SLM 21 is hereinafterreferred to as a “routing SLM”. Although the corrective SLM 20 is showndisposed upstream of the routing SLM 21, it may alternatively bedisposed downstream of the routing SLM 21, between two routing SLMs, orwith systems using routing devices other than the routing SLM 21.

The routing SLM 21 has operating circuitry 23 receiving routing controldata at a routing control input 24, and generating at the SLM 21 sets ofholograms for routing the beams 1,2. The corrective SLM 20 has operatingcircuitry 26 receiving compensation or adaptation data at a controlinput 27 to cause the SLM 20 to display selected holograms. In thisembodiment, the SLM 20 forms a reflective lens.

Synthesising a lens at the SLM 20 can be used to change the position ofthe beam focused spot and therefore correct for a position error ormanufacturing tolerance in one or more other lenses or reflective (asopposed to transmissive) optical elements, such as a curved mirror. Thesynthesised lens can be spherical or aspheric or cylindrical or asuperposition of such lenses. Synthesised cylindrical lenses may havearbitrary orientation between their two long axes and the lens focallengths can both be positive, or both be negative, or one can bepositive and the other negative.

To provide a desired phase modulation profile for a lens or curvedmirror to compensate for an unwanted deviation from a required systemcharacteristic, the system is modelled without the lens/mirror. Then alens/mirror having the correction to cancel out the deviation issimulated, and the parameters of the lens/mirror are transformed so thatwhen applied to an SLM the same effect is achieved.

In one application what is required is to adjust the position and widthof the beam waist, of a Gaussian-type beam at some particular point inthe optical system, in order to compensate for temperature changes orchanges in routing configuration. Hence two properties of the beam mustbe adjusted and so it is necessary to change two properties of theoptical system. In a conventional static optical system both a lensfocal length and the position of the lens are selected to achieve therequired beam transformation. In the dynamic systems under considerationit is rarely possible deliberately to adjust the position of the opticalcomponents. A single variable focus action at a fixed position changesboth the position and the width of the beam waist and only in specialcircumstances will both properties be adjusted to the required value.

One method to overcome this problem is to apply both corrective phaseand corrective ‘pseudo-amplitude’ modulation (to be described later)with a single SLM. However the amplitude modulation reduces the beampower which may be undesirable in some applications. A further andpreferred method is to apply corrective phase modulation with twoseparate SLMs.

For example consider coupling from one input fibre (or input beam)through a routing system into the selected output fibre (or outputbeam). Inside the routing system there are at least two SLMs carryingout a corrective function. They may also be routing and carrying outother functions (to be described in this application). In between agiven pair of SLMs carrying out focus correction there is anintermediate optical system.

At the first SLM carrying out a corrective function there may becalculated and/or measured the incident amplitude and phase distributionof the input beam that had propagated from the input fibre or beam. Atthe second SLM carrying out a corrective function there may becalculated and/or measured the ideal amplitude and phase distributionthat the output beam would adopt if coupling perfectly into the outputfibre or beam. This can be achieved by backlaunching from the outputfibre or beam or by a simulation of a backlaunch. The required focuscorrection functions of these two SLMs is to transform the incidentamplitude and phase distribution arriving at the first SLM to the idealamplitude and phase distribution at the second SLM to achieve perfect(or the desired) coupling efficiency into the output fibre or outputbeam.

The corrective phase modulation to be applied at the first SLM should becalculated, so as to achieve the ideal amplitude distribution at thesecond SLM as the beam arrives at the second SLM after passing from thefirst SLM and through the intermediate system. This calculation shouldtake into account propagation through the intermediate system betweenthe first and second SLMs. Hence the function of the first SLM is tocorrect the beam so as to achieve the ideal amplitude distribution forthe output beam. The beam phase distribution should also be calculatedas it arrives at the second SLM. The corrective phase distribution to beapplied at the second SLM should be calculated so as to transform thephase distribution of the beam incident upon it from the intermediatesystem to the ideal phase distribution required for the output beam atthe second SLM.

Two variables available at the SLM to effect corrections from an optimalor other desired level of performance are firstly the blocks of pixelsthat are delineated for the incident light beam, and secondly thehologram that is displayed on the block(s) of concern.

Starting with the delineation of blocks, it should be borne in mind thatthe point of arrival of light on the SLM can only be predicted to acertain accuracy and that the point may vary according to physicalchanges in the system, for example due to temperature effects or ageing.Thus, the device allows for assessment of the results achieved by thecurrent assignment, and comparison of those results with a specifiedperformance. In response to the comparison results, the delineation maybe varied so as to improve the results.

In one embodiment a training phase, uses for example a hill climbingapproach to control and optimise the position of the centre of theblock. Then if the “in-use” results deviate by more than a specifiedamount from the best value, the delineation of the block is varied. Thisprocess reassignment may step the assigned block one pixel at a time indifferent directions to establish whether an improved result isachieved, and if so continuing to step to endeavour to reach an optimumperformance. The variation may be needed where temperature effects causepositional drift between components of the device. It is important torealise that unlike MEMS systems and the like, all the pixels arepotentially available for all the beams. Also the size, shape andlocation of a delineated block is not fixed.

Equally the size and shape of a block may be varied if required. Suchchanges may be necessary under a variety of situations, especially wherea hologram change is needed. If for example a hologram requiring alarger number of pixels becomes necessary for one beam, the size of theblock to display that hologram can be altered. Such changes must ofcourse usually be a compromise due to the presence of other blocks(possibly contiguous with the present block) for displaying hologramsfor other beams of light.

Monitoring techniques for determining whether the currently assignedblock is appropriate include the techniques described later herein as“taking moments”.

Turning to variation of the hologram that is displayed on the block ofconcern, one option to take into account for example physical changes inthe system, such as movement out of alignment, is to change one normallinear-type routing hologram for another, or to adjust the presenthologram in direct response to the sensed change. Thus if, due forexample to temperature effects, a target location for a beam moves, itmay be necessary to change the deflection currently being produced at apixel block. This change or adjustment may be made in response to sensedinformation at the target location, and may again be carried out“on-line” by varying the hologram step by step. However, it may bepossible to obtain an actual measure of the amount and direction ofchange needed, and in this case either a new hologram can be read in tothe SLM or a suitable variation of the existing hologram carried out.

As well as, or instead of, linear changes to linear routing holograms,corrective changes may be needed, for example to refocus a beam or tocorrect for phase distortion and non-focus aberrations.

Having corrected the beam focus other aberrations may remain in thesystem. Such aberrations distort the phase distributions of the beams.These aberrations will also change with routing configuration as thebeams are passing through different lenses and/or different positions onthe same lenses. Similarly the aberrations will change with temperature.To obtain stable and acceptable performance of a reconfigurable opticalsystem, the aberrations can be corrected dynamically.

To provide a desired phase modulation profile for these aberrations thesystem may be modelled or measured to calculate the phase distortionacross the SLM, compared to the ideal phase distribution. The idealphase distribution may again be found by modelling the system‘backwards’ from the desired output beam, or by backlaunching andmeasurement, while the actual phase distribution may be found bymodelling the system forwards from the input beam or measurement. Thecalculations will include the effects of reflection from the SLM itself.The corrective function of the SLM is to transform between the actualand ideal phase distortion. The phase distortion is defined as the phasedifference between the actual phase distribution and the ideal phasedistribution. The desired corrective profile is the conjugate phase ofthe phase distortion.

Alternatively, these corrective functions can be shared by two SLMS,which allows an extra degree of freedom in how the beam propagatesinside the intermediate system between the two SLMs.

Further, given a real system a sampling method (as will be describedlater) may be used to direct a fraction of the beam towards a wavefrontsensor that may assess the beam. So far the process is deterministic.Then the changes are applied to the real system, and perturbations onthe parameters are applied while monitoring the sensor and/or theinput/output state, so as to determine whether an optimum configurationis achieved. If not, the parameters are changed until a best case isachieved. Any known optimising technique may be used. It is preferred toprovide a reasonable starting point by deterministic means, as otherwiselocal non-optimum performance maxima may be used instead of the trueoptimum.

The method or device of the invention may be used to correct foraberrations such as field curvature in which the output ‘plane’ of theimage(s) from an optical system is curved, rather than flat.

Equally, even if in use the SLM forms a corrective element by havingnon-linear phase modulation applied across it, if it is operated inseparate training and use phases, it may be desirable while training forthe SLM to route as well. In this case the SLM scans the processed beamover a detector or routes the beam, for example using one or more dummyholograms, into a monitor fibre.

Referring now to FIG. 4, the corrective SLM 20, used purely forsynthesising a corrective element, has operating circuitry 125, andfurther comprises processing circuitry 122 and temperature sensors 123.In this embodiment the operating circuitry, temperature sensors andprocessing circuitry are integrated on the same structure as the rest ofthe SLM, but this is not critical to the invention. Associated with theprocessing circuitry is a store 124 into which is programmed a lookuptable. The sensors detect temperature changes in the system as a wholeand in the SLM, and in response to changes access the look up table viathe processing circuitry 122 to apply corrections to the operatingcircuitry. These corrections affect the holograms displayed on theblocks 13, 14 of pixels. The sensors may also be capable of correctionfor temperature gradients.

This technique may also be applied to an SLM used for routing.

Referring now to FIG. 5, an optical system 35 has a corrective SLM 30with operating circuitry 31, and processing circuitry 32. The systemincludes further devices, here second and third SLMs 33 and 34, disposeddownstream of the corrective SLM 30. The second SLM 33 is intended toroute light to particular pixel groups 15, 16 of the third SLM 34. Thethird SLM 34 has monitor sensors 37 for sensing light at predeterminedlocations. In one embodiment these sensors 37 are formed by making thereflective layer partially transmissive, and creating a sensingstructure underneath. In another, the pixel electrode of selected pixelsis replaced by a silicon photodetector or germanium sensor structure.

In either case, circuitry may be integrated into the silicon backplaneto process the output of the sensors 37, for example to compare theoutputs of adjacent sensors 37, or to threshold one sensor againstneighbouring sensor outputs. Where possible, processing circuitry is onchip, as it is possible to reduce the time taken after light has beenreceived to respond to it in this way. This is because there is no needto read information off-chip for processing, and also becausecalculations may be able to be performed in parallel.

Provided the routing-together with any compensation effects from thecorrective SLM 30—is true, the sensors 37 will receive only a minimalamount of light. However where misalignment or focus errors are present,the extent of such errors is detected by measuring the power coupledinto the monitor sensors. To that end, the sensors 37 provide data,possibly after some on-chip processing, to the processing circuitry 32.The processing circuitry 32 contains a control algorithm to enable it tocontrol the operating circuitry 31 to make changes of, or adjustmentsto, the compensating holograms displayed on the corrective SLM 30 untilthe system alignment is measured to be optimised. In some embodiments,changes to the sub-arrays to which beam affecting holograms are appliedmay be made in response to the sensor output data.

In another embodiment a determined number of dummy ports are provided.For example for a connector two or more such ports are provided and forrouting devices three or more dummy ports are provided. These are usedfor continuous misalignment monitoring and compensation, and also forsystem training at the start.

Although some embodiments can operate on a trial and error basis, or canbe adapted “on the fly”, a preferred optical system uses a trainingstage during which it causes to be stored in the look-up table dataenabling operation under each of the conditions to be encountered inuse.

In one embodiment, in the training stage, a set of initial startingvalues is read in for application to the SLM 30 as hologram data, thenlight is applied at a fibre and the result of varying the hologram isnoted. The variations may include both a change of pixels to which thehologram is applied, and a change of the hologram. Where more than onefibre is provided, light is applied to each other fibre in turn, andsimilar results obtained. Then other environmental changes are appliedand their effects noted, e.g. at the sensors 37, and the correction forinput data either calculated or sought by varying the presently-applieddata using optimisation techniques to seek best or acceptableperformance

Then, in use, the system may be operated on a deterministic basis—i.e.after ascertaining what effect is sought, for example responding to atemperature change or providing a change in routing, the change to theapplied data for operating the device can be accessed without the needfor experiment.

A preferred embodiment operates in the deterministic way, but uses oneor more reference beams of light passed through the device using the SLM30. In that way the effect of deviations due to the device itself can beisolated. Also it can be confirmed that changes are being correctly madeto take into account environmental and other variations.

The device may also have further monitor sensors placed to receive thezero-order reflections from the SLM(s) to enable an assessment to bemade of the input conditions. For example, where an input channel fails,this can be determined by observing the content of the specularreflection from the light beam representing that channel. Where thereare two SLMs as in some routing systems, the specular reflections fromeach SLM may be sensed and compared.

Referring now to FIG. 6, a dual-function SLM 40 provides both routingand correction. The SLM 40 has operating circuitry 41 and processingcircuitry 42. The operating circuitry 41 receives routing data at afirst control input 44 for causing the processing circuitry 42 togenerate the holograms on the SLM 40 to achieve the desired routing. Theprocessing circuitry 42 also receives routing data on an input 45, andcontrols the operating circuitry 41 using an algorithm enablingadaptation due to changes in the path taken through the system to takeplace on a timescale equivalent to that required to change the hologramdisplay, i.e. of the order of milliseconds.

The control algorithms for this embodiment may use one or more ofseveral types of compensation.

In one embodiment a look-up table is stored in a memory 43, the look-uptable storing pre-calculated and stored values of the compensation foreach different route through the system.

In another embodiment the system is trained before first being operated,using changes of, or to the compensating holograms to learn how changingthe compensating holograms affects the system performance, the resultingdata being held in the memory 43.

In a further embodiment, the processing circuitry 42 employsintelligence responsive to signals from monitor sensors 47,48 formonitoring and calculation of how these compensating holograms shouldadapt with time to accommodate changes in the system alignment. This isachieved in some embodiments by integrating circuitry components intothe silicon backplane of the SLM, or by discrete components such asgermanium detectors where the wavelengths are beyond those attainable bysilicon devices. In some embodiments sensors 47 are provided for sensinglight at areas of the SLM, and in others the sensors 48 may instead oralso be remote from the SLM 40 to sense the effects of changes on theholograms at the SLM 40.

Referring now to FIG. 7, an optical system 80 includes an SLM 81 forrouting beams 1,2 of light from input fibres 3,4 to output fibres 5,6 bymeans of holograms displayed on pixel groups 13,14 of the SLM. Theholograms are generated by processing circuitry 82 which responds to acontrol input 83 to apply voltages to an array of pixellated elements ofthe SLM, each of which is applied substantially uniformly across thepixel of concern. This result is a discrete approximation of a linearphase modulation to route the beams.

The processing circuitry 82 calculates the ideal linear phase ramp toroute the beams, on the basis of the routing control input 83 andresolves this phase modulo 2Pi. The processing circuitry at each of thepixels then selects the closest available phase level to the idealvalue. For example if it is desired to route into the m′th diffractionorder with a grating period Ω the ideal phase at position u on the SLM81 is 2pi·mu/Ω. Therefore, approximately, the phase goes linearly fromzero up to 2pi over a distance Ω/m after which it falls back to zero,see FIG. 8 a.

Control of the power in individual wavelength channels is a commonrequirement in communication systems. Typical situations are the need toavoid receiver saturation, to maintain stable performance of the opticalamplifiers or to suppress non-linear effects in the transmission systemsthat might otherwise change the information content of the signals.Power control may be combined with sampling or monitoring channels toallow adjustment of the power levels to a common power level (channelequalisation) or to some desired wavelength characteristic.

Deliberate changes to the value of ‘Ω’ can be used to reduce thecoupling efficiency into the output in order to provide a desiredattenuation. This is suitable for applying a low attenuation. However,it is not suitable for a high attenuation as, in that event, the beammay then be deflected towards another output fibre, increasing thecrosstalk. If there is only one output fibre this method may be usedregardless of the level of attenuation.

To provide a selected desired attenuation of the optical channel in thesystem, processing circuitry 85 responds to an attenuation control input84 to modify the operation of the operating circuitry 83 whereby theoperating circuitry selects a linear phase modulation such that by theend of each periodic phase ramp the phase has reached less than 2pi, seeFIG. 8 b.

This may be achieved by calculating the ideal value of phase for everypixel, and then multiplying this ideal value by a coefficient r between0 and 1, determined on the basis of the desired attenuation. Thecoefficient is applied to every pixel of the array in order to get areduced level per pixel, and then the available phase level nearest tothe reduced level is selected.

The method of this embodiment reduces the power in this diffractionorder by making the linear phase modulation incomplete, such that by theend of each periodic phase ramp the phase has only reached 2pi·r. It hashowever been found that the method of this embodiment may not providesufficient resolution of attenuation. It also increases the strength ofthe unwanted diffraction orders likely to cause crosstalk. When combinedwith deliberate changes in the length of the ideal phase ramp theresolution of attenuation may be improved. Again if there is only asingle output fibre the crosstalk is less important.

Resolution may also be improved by having a more complex incompletelinear phase modulation. However, the unwanted diffraction orders maystill remain too strong for use in a wavelength-routed network. Hence tocontrol the power by adapting the routing hologram may have undesirableperformance implications in many applications, as crosstalk worsens withincrease of attenuation. The problem can be overcome by use of a complexiterative design. This could be used to suppress the higher orders butmakes the routing control more expensive.

Referring now to FIG. 9, a system 99 includes an SLM 90 controlled byapplying a discrete approximation of a linear phase modulation to routebeams 1,2 from input fibres 3,4 to output fibres 5,6 as previouslydescribed with respect to FIG. 7. Thus operating circuitry 91 selects arouting hologram for display by the SLM, in accordance with a routinginput 92, whereby the beams may be correctly routed, using a look uptable or as otherwise known. A memory holds sets of data each allowingthe creation of a respective power controlling hologram. Processingcircuitry 93 runs an algorithm which chooses a desired power controllinghologram corresponding to a value set at a power control input 94. Thepower controlling hologram is selected to separate each beam intorespective main 1 a, 2 a and subsidiary 1 b, 2 b beams, such that themain beams 1 a, 2 a are routed through the system and the or eachsubsidiary beam(s) 1 b, 2 b is/are diffracted out of the system, forexample to a non-reflective absorber 97.

The processing circuitry 93 applies the power controlling hologram datato a second input 95 of the operating circuitry 91 which acts on therouting hologram data so as to combine the routing and power controllingholograms together to provide a resultant hologram. The operatingcircuitry then selects voltages to apply to the SLM 90 so that the SLMdisplays the resultant hologram.

Thus power in a routing context is controlled by combining the routinghologram with another hologram that has the effect of separating thebeam into a main beam and a set of one or more subsidiary beams of thesethe main beam is allowed to propagate through the system as requiredwhile the other(s) are diffracted out of the system.

For example consider a hologram that applies phases of +φ and −φ onadjacent pixels. In terms of real and imaginary parts this hologram hasthe same real part, cos φ, on every pixel, see FIG. 10, while theimaginary part oscillates between +−sin φ. It can be shown using Fouriertheory that the net effect is to multiply the amplitude of the originalrouted beam by a factor cos φ, and to divert the unwanted power into aset of weak beams at angles that are integer multiples of ±λ/2p withrespect to the original routed beam, where λ is the operating wavelengthand p is the pixel pitch.

The system is designed from a spatial viewpoint such that lightpropagating at such angles falls outside the region of the output fibres5,6 of FIG. 9. An alternative design directs the unwanted light intooutput fibres 5,6 at such a large angle of incidence that the couplinginto the fundamental mode is very weak, and has no substantial effect.In this case the unwanted power is coupling into the higher-order modesof the fibre and so will be attenuated rapidly. A fibre spool or someother technique providing mode stripping is then used on the outputfibre before the first splice to any other fibre.

In either case, the effective attenuation of the beam is 10 log₁₀ cos²φ.Hence, in this way, polarisation-independent phase modulation may beused to create an effect equivalent to polarisation-independentamplitude modulation. This is termed herein “pseudo amplitudemodulation”. In this particular case the pseudo-amplitude modulationapplied at every pixel is cos φ.

It will be clear to those skilled in the art that use of alternatepixels as the period of alternation is not essential, and may in somecases be undesirable. This is because of edge effects in the pixels.

The period and pattern of alternation can be varied so as to adjust thedeflection angle of the ‘unwanted power’. This light directed away fromthe output fibres can be collected and used as a monitor signal. Hencethe pseudo-amplitude modulation can be used to sample the beam incidenton an SLM as previously discussed. This sampling hologram can becombined with a routing and/or power control and/or corrective SLM. Inthe latter case the sampled beam can be directed towards a wavefrontsensor and then used to assess the quality of the beam correction. Whilethe pseudo-amplitude modulation as described above is applied to thewhole beam, it could be applied selectively to one or more parts of thebeam.

A further modification to this pseudo-amplitude modulation is tomultiply it by a further phase modulating hologram such as to achieve anet effect equivalent to a complex modulation.

It is often important that the sampling hologram takes a true sample ofthe output beam. Therefore in some cases the sampling hologram should beapplied after the combination of all other desired effects includingresolution modulo 2pi and approximation to the nearest available phaselevel. In this case the overall actual phase modulation distribution isachieved by a method equivalent to forming the product of the samplinghologram and the overall hologram calculated before sampling.

Similar pseudo-amplitude modulation techniques may be extended tosuppress the crosstalk created by clipping of the beam tails at theedges of each hologram and to tailor the coupling efficiency vs.transverse offset characteristic of the output fibres. Since thetransverse position at the output fibre is wavelength dependent, thistailoring of the coupling efficiency vs. offset can be used to tailorthe wavelength response of the system. This is important in the contextof wavelength division multiplexing (WDM) systems where the systemwavelength can be expected to lie anywhere in the range of the availableoptical amplifiers. The output angle for beam steering using an SLM andperiodic linear phase modulation is proportional to the wavelength whilethe focal length of corrective lenses is also wavelength-dependent.Therefore a hologram configured to give the optimum coupling efficiencyat one wavelength will produce an output beam with transverse and/orlongitudinal offset at another wavelength. These effects result inwavelength-dependent losses in systems required to route many wavelengthchannels as an ensemble. Hence a method designed to flatten orcompensate for such wavelength-dependent losses is useful and important.

Among the envisaged applications are the flattening of the overallwavelength response and the compensation for gain ripple in opticalamplifiers, especially Erbium-doped fibre optic amplifiers (EDFA).

An SLM device may also be used to adapt the shape, e.g. the mode fieldshape, of a beam in order to suppress crosstalk.

Beam shaping is a type of apodisation. It is advantageously used toreduce crosstalk created at a device by clipping of the energy tails ofthe light beams. Such clipping leads to ripples in the far field. Theseripples cause the beam to spread over a wider region than is desired. Intelecommunications routing this can lead to crosstalk. Otherapplications may also benefit from apodisation of a clipped laser beam,such as laser machining, for example, where it is desired to process aparticular area of a material without other areas being affected andlaser scalpels for use in surgery.

Clipping occurs because the energy of the beam spreads over an infiniteextent (although the amplitude of the beam tails tends to zero), whileany device upon which the beam is incident has a finite width. Clippingmanifests itself as a discontinuity in the beam amplitude at the edgesof the device.

Referring to FIG. 11, two SLMs 100,101 are used for beam steering orrouting of beams 1,2 from input fibres 3,4 to output fibres 5,6, asdescribed in PCT GB00/03796. Each SLM 100,101 is divided into a numberof blocks of pixels 103 a, 104 a; 103 b, 104 b. Each block 103 a, 104 ais associated with a particular input fibre 3,4—i.e. the fibre ofconcern points to the subject block. Each block displays a hologram thatapplies routing. As previously discussed herein the holograms may alsoor alternatively provide focus compensation, aberration correctionand/or power control and/or sampling, as required.

The blocks 103 a, 104 a at the input SLM 100 each receive a beam from anassociated input fibre 3,4 while the blocks 103 b, 104 b at the outputSLM 101 each direct a beam towards an associated output fibre 5,6. Eachblock 103 a, 103 b has a finite width and height. As known to thoseskilled in the art and as previously noted, the beam width is infinite,therefore the block clips the beam from or to the associated fibre andthis creates undesired ripples in the far field.

The ripples due to clipping of the beam 1 are figuratively shown asincluding a beam 106 which, it will be seen, is incident on the wrongoutput hologram, displayed on block 104 b at the output SLM 101. “Wrong”signifies holograms other than that to which the beam of concern isbeing routed, for example holograms displayed by blocks around the blockto which the beam should be routed. Some of these ripples will then becoupled into “wrong” output fibres 5,6—i.e. those to which the beam isnot deliberately being routed-leading to crosstalk. It will be clear tothose skilled in the art that these effects will be present on blocksother than those adjacent to the “correct” blocks, as the field of beam1 is infinite in extent.

In any physical system the effect of the ripples created by clipping atthe output SLM 101 depends on the optical architecture.

In practice the non-ideal transfer function of the optics (due to finitelens apertures and aberrations) means that a sharp change in theamplitude spreads out and causes crosstalk in adjacent output fibres. Ineffect the optics applies a limit to the range of spatial frequenciesthat can be transmitted. This frequency limit causes crosstalk.

The wider the device, compared to the beam spot size at the device, theweaker the ripples in the far field and the lower the crosstalk. Ingeneral a parameter C is defined such that the required width of SLM perbeam is given by H=C·ω, where ω is the beam spot size at the SLM. Thevalue of C depends on the beam shape, the optical architecture and theallowable crosstalk. Typically for a Gaussian beam, with no beam shapingand aiming for crosstalk levels around −40 dB, C would be selected tohave a value greater than or equal to three. Looking at this system fromthe spatial frequency viewpoint, the field incident on the SLM contains(for perfect optics) all the spatial frequencies in the input beam. Thefinite device width cuts off the higher spatial frequencies, so, again,the optics applies a limit to the range of spatial frequencies that canbe transmitted and this frequency limit causes crosstalk.

Beam shaping can be used to decrease the crosstalk for a given value ofC, and also allow the use of a lower value of C. Calculations for N*Nswitches have shown that decreasing the value of C leads to more compactoptical switches and increases the wavelength range per port. Hence beamshaping can be employed to provide more compact optical switches and/oran increased wavelength range per port.

The idea behind using beam shaping or ‘apodisation’ to reduce crosstalkis based on an analogy with digital transmission systems. In thesesystems a sequence of pulses is transmitted through a channel possessinga limited bandwidth. The frequency response of the channel distorts theedges of pulses being transmitted so that the edges may interfere withone another at the digital receiver leading to crosstalk. The channelfrequency response can, however, be shaped so as to minimise suchcrosstalk effects. Filters with responses that have odd-symmetry can beused to make the edges go through a zero at the time instants whenpulses are detected.

Therefore beam-shaping with odd symmetry can be used to make thecrosstalk go through a zero at the positions of the output fibres. Sucha method is likely to be very sensitive to position tolerances.

Another method used in digital systems is to shape the frequency cut-offso that it goes smoothly to zero. In the present context the ideal caseof ‘smoothly’ is that the channel frequency response and all derivativesof the frequency response become zero. In practice it is not possible tomake all derivatives go to zero but a system may be designed in whichthe amplitude and all derivatives up to and including the k′thderivative become zero at the ends of the frequency range. The higherthe value of k, the quicker the tails of the pulse decay. Therefore thebeam shaping should go as smoothly as possible to zero.

To investigate the effects of beam shaping the amplitude modulation wastreated as continuous. The system studied was a single lens 2 f systemwhere 2 f is the length of the system between fibres and SLM, assuming fis the focal length with fibres in one focal plane, and an SLM in theother focal plane. The input fibre beam was treated as a Gaussian.Various amplitude modulation shapes were applied at the SLM and thecoupling efficiency into the output fibre was calculated. In thisarchitecture and from Abbe theory, the incident field at the SLM isproportional to the Fourier Transform of the field leaving the inputfibre. In particular, different spatial frequencies in the fibre modeland on different parts of the SLM. Clipping removes the spatialfrequencies outside the area of the hologram. Beam shaping at the SLMhas the effect of modifying the relative amplitude of the remainingspatial frequencies.

Residual ripples may still remain due to the discontinuity in the beamderivative but the ripples will be reduced in amplitude and decay morequickly. Further reduction in the ripple amplitude and increase in therate of decay may be achieved by shaping the beam such that both theamplitude and the first k derivatives go to zero at the edges.

Mathematical analysis of the effect has also been carried out. Theresults are as follows:

The n^(th) time derivative of a function can be expressed in terms ofits Fourier Transform as shown in equation (1):

$\begin{matrix}{\frac{^{n}{g(t)}}{t^{n}} = {\int_{- \infty}^{\infty}{\left( {i\; 2\pi \; f} \right)^{n}{G(f)}\exp \; i\; 2\pi \; {ft}\ {f}}}} & (1)\end{matrix}$

Hence, by inversion, the frequency dependence of the Fourier Transform(FT) may be expressed as an FT of any one of the function's derivativesas shown in equation (2):

$\begin{matrix}{{G(f)} = {{\frac{1}{\left( {i\; 2\pi \; f} \right)^{n}}{\int_{- \infty}^{\infty}{\frac{^{n}{g(t)}}{t^{n}}\exp}}} - {i\; 2\pi \; {ft}\ {t}}}} & (2)\end{matrix}$

Choosing the zeroth derivative provides the expression in equation (3):

$\begin{matrix}{{G(f)} = {{\int_{- \infty}^{\infty}{{g(t)}\exp}} - {i\; 2\pi \; {ft}\ {t}}}} & (3)\end{matrix}$

To apply the analysis to free-space beam-steering:

let x and y be the position co-ordinates at the fibre output from aswitch, and u and v be the position co-ordinates at the SLM. Assume theSLM to be in one focal plane of a lens of focal length f, and the fibrearray to be in the other focal plane:

$\begin{matrix}{{E_{FIB}\left( {x,y} \right)} = {\frac{i}{f\; \lambda}{\exp\left( {{- i}\frac{2\pi}{\lambda}\left( {{2f} + {nt}} \right)} \right)}{\int{\int{{E_{SLM}\left( {u,v} \right)}\exp \; i\frac{2\pi \; f}{\lambda}\left( {{xu} + {yv}} \right){u}{v}}}}}} & (4)\end{matrix}$

such that the output field (see equation (4)) is a 2-D Fourier Transformof the field at the SLM, E_(SLM). In this result t is the lens thicknessand N its refractive index, while λ is the optical wavelength.

For the present purposes the 1-D equivalent is considered (relation 5):

$\begin{matrix}{{E_{FIB}(x)} = {\frac{i}{f\; \lambda}{\exp\left( {{- i}\frac{2\pi}{\lambda}\left( {{2f} + {nt}} \right)} \right)}{\int{{E_{SLM}(u)}\exp \; i\frac{2\pi \; f}{\lambda}({xu}){u}}}}} & (5)\end{matrix}$

Comparing with (3) it is clear that the position co-ordinate at the SLM(u) is equivalent to the time domain and the position co-ordinate at theoutput (x) is equivalent to the frequency domain. Hence from (2) theoutput field may be expressed in terms of a derivative of the field atthe SLM, as shown in equation (6):

$\begin{matrix}{{E_{FIB}(x)} = {\frac{i}{f\; \lambda}{\exp\left( {{- i}\frac{2\pi}{\lambda}\left( {{2f} + {nt}} \right)} \right)}\left( \frac{i}{2\pi \; x} \right)^{n}{\int{\frac{^{n}{E_{SLM}(u)}}{u^{n}}\exp \; i\frac{2\pi \; f}{\lambda}({xu}){u}}}}} & (6)\end{matrix}$

Let the k^(th) derivative of E_(SLM)(u) be non-zero and smoothly varyingover the range [−H/2, H/2], but zero outside this range, such that thederivative changes discontinuously at u=±H/2:

$\begin{matrix}\begin{matrix}{\frac{^{k}{E_{SLM}(u)}}{u^{k}} = 0} & {{\forall{u:{u < {- \frac{H}{2}}}}}} \\{= g^{H}} & {{u = {- \frac{H}{2}}}} \\{= {{s(u)} + g^{H}}} & {{{- \frac{H}{2}} < u < \frac{H}{2}}} \\{= g^{H}} & {{u = {+ \frac{H}{2}}}} \\{= 0} & {{u > \frac{H}{2}}}\end{matrix} & (7)\end{matrix}$

This representation assumes E_(SLM) to be even in u. Physically thissituation represents a beam that is perfectly aligned with respect tothe centre of a hologram of width H.

This derivative may be expressed as the sum of a rect function and asmoothly varying function, s(u), that is zero at and outside |u|=H/2:

$\begin{matrix}{\frac{^{k}{E_{SLM}(u)}}{u^{k}} \equiv {{g_{H}{{rect}\left( \frac{u}{H/2} \right)}} + {s(u)}}} & (8)\end{matrix}$

For example consider a clipped (and unapodised) Gaussian beam; thezeroth derivative (k=0) may be expressed as:

$\begin{matrix}\begin{matrix}{{s(u)} = {\exp - \left( \frac{u}{\omega_{HOL}} \right)^{2} - \exp - {\left( \frac{H}{2\omega_{HOL}} \right)^{2}{\forall{{u} < \frac{H}{2}}}}}} \\{= {0\mspace{14mu} {\forall{{u} \geq \frac{H}{2}}}}}\end{matrix} & (9) \\{g_{H} = {\exp - \left( \frac{H}{2\omega_{HOL}} \right)^{2}}} & (10)\end{matrix}$

Now returning to the general case (equation (8)) the k+1^(th) derivativeis calculated:

$\begin{matrix}{\frac{^{k + 1}{E_{SLM}(u)}}{u^{k + 1}} \equiv {{g_{H}\left\{ {{\delta \left( {u + \frac{H}{2}} \right)} - {\delta \left( {u - \frac{H}{2}} \right)}} \right\}} + \frac{{s(u)}}{u}}} & (11)\end{matrix}$

It is now convenient to calculate the output field. Set n=k+1 in (6) toobtain:

$\begin{matrix}{{E_{FIB}(x)} \propto {\frac{1}{\left( {i\; 2\pi \; x} \right)^{k + 1}}\begin{Bmatrix}{{g_{H}{\int_{- \infty}^{\infty}{\left( {{\delta \left( {t + {H/2}} \right)} + {\delta \left( {t - {H/2}} \right)}} \right)\exp}}} - {i\; 2\pi \; {xu}\ {u}} +} \\{{\int_{- \infty}^{\infty}{\frac{{s(u)}}{u}\exp}} - {i\; 2\pi \; {xu}\ {u}}}\end{Bmatrix}}} & (12)\end{matrix}$

which becomes equation (13):

$\begin{matrix}{{E_{FIB}(x)} \propto {\frac{1}{\left( {i\; 2\pi \; x} \right)^{k + 1}}\left\{ {{2\; {ig}_{H}{\sin \left( {\pi \; {xH}} \right)}} + {\int_{- \frac{H}{2}}^{\frac{H}{2}}{\frac{{s(u)}}{u}\exp}} - {i\; 2\pi \; {xu}\ {u}}} \right\}}} & (13)\end{matrix}$

As the position is increased, the exponential term in the 2^(nd)integral of (13) oscillates more and more rapidly. Eventually thespatial frequency is so high that the derivative of s(u) can beconsidered to be constant, or nearly constant, over the spatial period.In which case the integral is zero, or nearly zero, when evaluated overeach period of the oscillation. Therefore at high frequencies the wholeof the second integral must approach zero.

It is assumed that the behaviour is dominated by the first integral. Thefirst integral shows that if the amplitude changes discontinuously (k=0,i.e. an unapodised hologram), the spectrum (E_(FIB)) decays as 1/x. Now,if the amplitude and the first derivative are continuous, it is thesecond derivative that changes discontinuously, and so k=2 and thespectrum (E_(FIB)) decays as 1/x³. Numerical simulations have beencarried out to confirm this behaviour.

A particularly advantageous shape is one in which the shaped beam hasodd symmetry about points midway between the centre and the edges suchthat the beam amplitude and all of its derivatives go to zero at thebeam edges.

The beam shaping may be effected to remove only a small amount of powerfrom the central portion of the beam, to maintain acceptable systemefficiency. A method for shaping a beam to achieve suppression of theripples is now described.

Defining the middle of the beam as f(u), then f(u) can describe theoriginal beam in its central portion, or what is left in the originalbeam after it has already been partially shaped, using, for example,pseudo-amplitude. To avoid ripples in the far field the edges of thebeam go to zero at u=±H/2, where H is the width of the hologram.

Hence, at the right-hand edge, describe the beam as in equation (14):

f _(R)(u)=f(0)−f(u−H/2)  (14)

(The left-hand edge is considered later).

To get matching of the amplitude half-way between the middle and theedge it is required that

f(H/4)=f _(R)(H/4)  (15)

From which there is obtained

f(H/4)+f(−H/4)=f(0)  (16)

Now consider the derivatives at the joining point. The n^(th) derivativeof the right-hand edge function is given by equation (17):

$\begin{matrix}{{\frac{^{n}f_{RH}}{u^{n}}_{u = U}} = {{- \frac{^{n}f}{u^{n}}}_{u = {U - {H/2}}}}} & (17)\end{matrix}$

Hence at the joining point condition (18) is valid:

$\begin{matrix}{{\frac{^{n}f_{RHEDGE}}{u^{n}}_{u = {H/4}}} = {{- \frac{^{n}f}{u^{n}}}_{u = {{- H}/4}}}} & (18)\end{matrix}$

In order to avoid the creation of high frequency effects (crosstalktails) by the joining point all derivatives are desirably continuoushere. Hence it is required that condition (19) should be true:

$\begin{matrix}{{\frac{^{n}f}{u^{n}}_{u = {H/4}}} = {{- \frac{^{n}f}{u^{n}}}_{u = {{- H}/4}}}} & (19)\end{matrix}$

To find out whether this is possible, expand the function f in a Taylorseries about x=0 to obtain equation (20)

f=f(0)+a ₁ u+a ₂ u ²+a₃ u ³+a₄ u ⁴+a₅ u ⁵+a₆ u ⁶+  (20)

The first derivative is given by equation (21):

$\begin{matrix}{\frac{f}{u} = {a_{1} + {2a_{2}u} + {3a_{3}u^{2}} + {4a_{4}u^{3}} + \ldots}} & (21)\end{matrix}$

The required condition (19) for the first derivative (n=1) can beobtained provided f is even in x, so that all the odd coefficients {a1,a3 . . . } in (20) and (21) are zero. This makes the first derivativecontinuous at the joining point. Furthermore if f is an even functionthen f(H/4)=f(−H/4) in which case (16) becomes:

$\begin{matrix}{{f\left( {H/4} \right)} = {\frac{1}{2}{f(0)}}} & (22)\end{matrix}$

Given that f is now an even function, the second derivative off is givenby equation (23):

$\begin{matrix}{\frac{^{2}f}{u^{2}} = {{2a_{2}} + {12a_{4}u^{2}} + \ldots}} & (23)\end{matrix}$

Returning to the required condition in (19) it is clear that it cannotbe satisfied for n=2. Hence the second derivative is discontinuous atthe joining point u=H/4.

The left-hand edge is given by equation (24)

f _(LH)(u)=f(0)−f(u+H/2)  (24)

Given that f is even, the overall function has odd symmetry in each halfplane x=±H/4.

To work out what happens at x=±H/2, expand f_(RH) and f_(LH) in Taylorseries, as shown in equations 25 and 26:

$\begin{matrix}{f_{RH} = {{a_{2}\left( {u - \frac{H}{2}} \right)}^{2} + {a_{4}\left( {u - \frac{H}{2}} \right)}^{4} + {a_{6}\left( {u - \frac{H}{2}} \right)}^{6} + \ldots}} & (25) \\{f_{LH} = {{a_{2}\left( {u + \frac{H}{2}} \right)}^{2} + {a_{4}\left( {u + \frac{H}{2}} \right)}^{4} + {a_{6}\left( {u + \frac{H}{2}} \right)}^{6} + \ldots}} & (26)\end{matrix}$

The function and its first derivative are both zero at u=½H, but thesecond derivative has the value 2a₂ Outside of the range [−½H, ½H] thebeam drops to zero. Hence the second derivative is discontinuous at bothu=±½H and u=±H/4, and the far field must therefore decay as the cube ofthe distance measured in the far field.

From the analysis, the required properties of f(u) for a hologram ofwidth H are that firstly it should be even in u, and that secondly itsamplitude at the position u=H/4 should be half the amplitude at u=0.After apodisation has been applied the shape of the beam in the regionbetween u=H/4 and u=H/2 should be given by f_(RH)(u)=f(0)−f(u−H/2) whilein the region between u=−H/2 and u=−H/4 the shape of the beam should begiven by fLH(u)=f(0)−f(u+H/2). In practice the shaping may not increasethe local beam amplitude. Hence the hologram width and/or the shape ofthe central portion may have to be adjusted to avoid the requirement for‘amplifying’ shaping.

As an example these conditions are satisfied by a Gaussian distributiongiven by equation (27):

$\begin{matrix}{{f(u)} = {\exp - \left( \frac{u\sqrt{\ln (2)}}{H/4} \right)^{2} -}} & (27)\end{matrix}$

If the original beam satisfies the first two conditions it can beapodised without removing power from the central region. Otherwiseshaping can be applied to the central region so that these twoconditions are satisfied.

In some systems there may be a requirement to adapt the width of thebeam in the far field: either to narrow the beam or to broaden the beam.This may be useful for laser processing of materials as well as forrouting. It is advantageous that the method to change the width does notintroduce side lobes. A particular application that would benefit islaser drilling of holes. The SLM could be used to narrow the drillingbeam as well as to change its focus so that the drilled hole remains ofuniform diameter (or has reduced diameter variation) as the hole isprogressively bored.

In order to broaden the far field, the near field (at the SLM) needs tobe made narrower. This may be implemented by applying shaping to thecentral portion of the beam so that its full width half maximum (FWHM)points become closer together and so that the beam shape has evensymmetry about its centre. Preferably the amplitude at the very peak isnot reduced so as not to lose too much power. The distance between thetwo FWHM points defines the effective half-width of the hologram.Further shaping should be applied to the left-hand and right-hand edgesof this effective hologram, so that the beam shape has the requiredproperties as described previously. Outside of the width of theeffective hologram the beam shape should have zero amplitude.

To narrow the far field, the near field (at the SLM) needs to be madebroader. This may be implemented by applying shaping to the centralportion of the beam, so that the FWHM points become further apart, andso that the beam shape has even symmetry about its centre. Typicallythis will require reduction of the amplitude around its peak. The extentof this reduction is governed by the need to be able to apply shaping tothe right and left hand edges of the hologram with the constraint thatthe shaping may only decrease the amplitude (and not increase it).

Amplitude-modulating SLMs can be used to implement the shaping but theyare polarisation-dependent.

Another pseudo-amplitude modulation can be created to implement the beamshaping by using a phase-modulating SLM, which may be madepolarisation-independent. This may be achieved by recognising that aphase modulation exp j φ(u), where j is the complex operator, isequivalent to a phase modulation cos φ(u)+j sin φ(u). Now choose φ(u)such that the modulus of φ(u) is varying slowly but the sign isoscillating.

Hence the real part of the modulation, cos φ(u), will be slowly varyingand can act as the amplitude modulator to create the beam shape, whilethe imaginary part of the modulation, ±sin φ(u), will be oscillatingrapidly with an equivalent period of two or more pixels. Hence theenergy stripped off by the effective amplitude modulator will bediffracted into a set of beams that are beam-steered out of the systemat large angles.

In a preferred embodiment, the system is designed such that lighttravelling at such angles will either not reach the output plane or willland outside the region defined by the output ports. Therefore the beamcomponent shaped by sin φ(u) is rejected by the optical system, whilethe beam component shaped by cos φ(u) is accepted by the system andcouples into one or more output ports, as required. While thisexplanation is for a one-dimensional phase modulator array the sameprinciple is applicable in 2-D. If φ(u) varies from 0 at the centre ofthe beam to π/2 at the edges then the amplitude of the beam shaped bycos φ(u) varies from 1 at the centre of the beam to 0 at the edges, thusremoving the amplitude discontinuity that creates rippling tails in thefar field. This can be achieved with minimal change to the insertionloss of the beam as it passes through the system. Indeed, often theinsertion loss due to clipping is due to interference from the amplitudediscontinuity, rather than the loss of energy from the beam tails.

The beam-shaping hologram is non-periodic but oscillatory and may beapplied as a combination with other routing and/or lens synthesis and/oraberration correcting and/or power control and/or sampling holograms.

Further advantages of the beam shaping are that it reduces the requiredvalue of C for a given required crosstalk, allowing more compact opticalswitches. Another advantage is that the crosstalk decays much morerapidly with distance away from the target output fibre. Hence,essentially, the output fibres receive crosstalk only from their nearestneighbour fibres.

Therefore in a large optical switch used as a shared N*N switch for arange of wavelengths, it should be possible to arrange the wavelengthchannel allocation such that no output fibre collects crosstalk from achannel at the same system wavelength as the channel it is supposed tobe collecting. This would reduce significantly the homodyne beat noiseaccumulation in networks using such switches, and, conversely, allow anincrease in the allowed crosstalk in each switch as heterodyne crosstalkhas much less of an impact at the receiver, and can also be filtered outif necessary.

The crosstalk suppression method uses beam shaping to suppress ripplesin the beam tails. The same method can be adapted to change the beamshape around the beam centre. For the case when the output beam is animage of the beam at the SLM the beam shaping is working directly on animage of the output beam. The fraction of the initial beam that isshaped by the slowly varying function cos φ(u) can have the correctsymmetry to couple efficiently into the fundamental mode of the outputfibre. The fraction of the initial beam that is shaped by the rapidlyvarying function±sin φ(u) has the wrong symmetry to couple into thefundamental mode and can be adjusted to be at least partially orthogonalto the fundamental mode.

Effectively, it is the fraction of the beam shaped by cos φ(u) thatdominates the coupling efficiency into the fundamental mode. Thereforethe dependence of the coupling efficiency vs. transverse offset isdominated by the overlap integral between the cos φ(u) shaped beam andthe fibre fundamental mode.

When the incident beam is the same shape as the fundamental mode and forsmall transverse offsets the coupling efficiency decreases approximatelyparabolically with transverse offset. In many beam-steering systemsusing phase-modulating SLMs the transverse offset at the output fibreincreases linearly with the wavelength difference from the designwavelength. Consequently the system coupling efficiency decreasesapproximately parabolically with wavelength difference from the designwavelength. Beam shaping can be used to adjust the shape of the incidentbeam and optimised to flatten the dependence on transverse offset andhence to flatten the wavelength response. Alternatively a more complexwavelength dependence could be synthesised to compensate for otherwavelength-dependent effects.

Beam shaping may also be used during system assembly, training oroperation in order to measure mathematical moments of a light beam. Adescription of the method and theory will be followed by a descriptionof some example applications.

The method requires a first stage during which corrective phasemodulation is applied by the SLM such that the phase profile of the beamleaving the SLM has no non-linear component. This may be confirmed witha collimeter or wavefront sensor or some other suitable device. In afirst embodiment the phase profile has no linear component applied todeflect the beam such that the beam is reflected in a speculardirection. An optical receiver is placed to receive the reflected beam.The power reflected exactly into the specular direction is proportionalto the square of an integral A(n) given in equation (28) where f(n,u,v)is the complex amplitude of the beam leaving the SLM at co-ordinates u,vduring the n^(th) stage of the method.

A(n)=∫∫f(n,u,v)du dv  (28)

The optical power received by the photodiode during the n^(th) stage ofthe method is given by equation (29)

P(n)=K(A(n))²  (29)

where K is a constant of proportionality

If received by an optical fibre the received power will be modifiedaccording to the fibre misalignment and mode field distribution, leadingto possible ambiguities in the method. Hence it is preferred instead toreceive the beam by a photodiode. During the first stage of the methodthe net phase modulation applied by the SLM is such that the beam is ofuniform phase. Let b(u, v) be the beam amplitude distribution. Thereforeduring this first stage the integral A is equal to the zeroth moment,a0, of the beam amplitude distribution, as shown in equation (30), andf(n,u,v) is equal to

A(1)=a0=∫∫b(u,v)du dv  (30)

Therefore the power, P(1), measured by the photodiode during this firststage is given by equation (31).

P(1)=Ka ₀ ²  (31)

In order to characterise a two-dimensional beam, moments of the beamdistribution may be taken in two orthogonal directions, in this case theu and v directions. Consider the pixel block of concern to be broken upinto a set of columns. To each column in the block a particulareffective amplitude modulation may be applied using the pseudo-amplitudemethod or some other method. For example consider the pixel column witha centre at co-ordinate u*. By applying an alternating phase modulationof +φ(u*) and −φ(u*) to adjacent pixels in the same column the effectiveamplitude modulation applied to the particular column is cos(φ(u*)).

In order to calculate the first moment in the u direction, during thesecond stage of the method the values of cos(φ(u*)) are chosen such asto approximate to a linear distribution, as described in equation (32)

cos(φ(u*))≈mu*+c  (32)

Therefore the power P(2) measured during the second stage of the processis given by (33).

P(2)≈K(m ² a _(1U) ²+2mca _(1U) a ₀ +c ² a ₀ ²)  (33)

where a_(1U) is the first moment of the beam distribution in the udirection, as given by (34).

a1u=∫∫ub(u,v)du dv  (34)

The ratio of the powers measured during the two stages is then given byequation (35)

$\begin{matrix}{\frac{P(2)}{P(0)} \approx {{m^{2}\left( \frac{a_{1U}}{a_{0}} \right)}^{2} + {2\mspace{14mu} m\; c\frac{a_{1U}}{a_{0}}} + c^{2}}} & (35)\end{matrix}$

Given the measured power ratio and the values of m and c as chosen tosatisfy the constraints of the method, the quadratic equation given in(35) may be solved to calculate the ratio of the first order moment inthe u direction to the zeroth order moment.

The constraints on m and c are such that the actual values of thealternating phase of each column need to be chosen from the availableset and such that the total phase excursion across the expected area ofthe beam remains within the range [0,π] or [−π,0] so that the cos(φ(u*))term may decrease (or increase) monotonically. In practise a photodiodeof finite size will receive power diffracted from the SLM within anangular distribution about the specular direction. A further constrainton the gradient ‘m’ in equation (32) is such that the side lobes createdby the linear amplitude modulation fall outside the area of thephotodiode.

Similar methods may be used to take approximate higher-order moments inthe u direction, and also first and higher-order moments in the vdirection. In the latter case to each row in the block a particulareffective amplitude modulation is applied, e.g. by setting adjacentpixels in the row to alternating phases of +φ(v*) and −φ(v*), where v*is the position co-ordinate of the row. The second-order moments mayalso be calculated and used to estimate the beam spot size at thehologram. This estimate can be used as part of the control algorithm forfocus adjustment.

In a second embodiment a further linear phase modulation is applied tothe hologram during each stage so as to deflect the beam to be measuredwhile taking the moments towards a particular photodiode.

Consider a Gaussian type beam b(u,v) centred at position co-ordinates(u0,v0). The even symmetry of the beam about axes parallel to the u andv directions and through the centre lead to the identities given byequations (36) and (37).

∫∫(u−u0)b(u,v)du dv=0  (36)

∫∫(v−v0)b(u,v)du dv=0  (37)

Hence approximate values of the first order moments measured asdescribed previously, or by some other method, may be used to deduceapproximate positions for the beam centres, as shown by equations (38)and (39).

$\begin{matrix}{u_{0} \approx \frac{a_{1U}}{a_{0}}} & (38) \\{v_{0} \approx \frac{a_{1V}}{a_{0}}} & (39)\end{matrix}$

In the next stage of the measurement the pixel block initially assignedto the beam is re-assigned such that it is centred within half a pixelin each of the u and v directions from the approximate centre of thebeam, as just calculated.

Let the new centre of the pixel block be at (u1,v1). A new hologramshould be calculated such that the beam leaving the SLM acts as theproduct of a beam of uniform phase distribution and an effectiveamplitude distribution given by equation (40)

cos(φ(u*))≈m(u*−u1)  (40)

The principle is that if the beam centre lies exactly at u1 the measuredpower exactly in the specular direction will be zero. Taking intoaccount the finite area of the photodiode the measured power cannot bezero but will be minimised when u1 is within half a pixel pitch of thebeam centre.

This new hologram should be applied to the pixel block and the powermeasured. At this point the method can proceed in two ways.

In one embodiment a further estimate of the beam centre can becalculated, as described previously, a new centre position u1calculated, the hologram recalculated according to equation (40) and thepower measured again. This process can be repeated until the value of u1appears to have converged.

In a second embodiment the centre of the pixel block, u1 can bere-assigned, the hologram recalculated according to (40) and the powermeasured again. At the current pixel block centre, u1, for which thebeam centre is within half a pixel of u1, the measured power should beat a minimum value.

A further embodiment is to use a suitable combination of these twoalternative methods.

The centre of the pixel block in the v direction can be measured usingsimilar methods.

The size of the pixel block used should be chosen so as to cover theexpected area of the beam. Outside of this area the phase can bemodulated on a checkerboard of, for example, +−pi/2, so that theeffective amplitude modulation is zero and the light from these regionsis diffracted far away from the photodiode.

It can be shown that equations (36) and (37) are also satisfied if thebeam waist is not coincident with the SLM, that is the beam isdefocused. Although the method as described above will not becalculating the proper moments of the beam, it can be shown that theposition of the beam centre may still be identified using the methodsdescribed.

The beam shaping method may be extended to control and adapt theamplitude of the beam steered through the system. If φ(u) varies from ψat the centre of the beam to π/2 at the edges then the real part of thepseudo-amplitude modulation can be considered as cos ψ multiplied by anideal beam-shaping function that causes insignificant insertion loss. Inwhich case there is an associated additional insertion loss given byapproximately 10 log₁₀ (cos² ψ). By varying the value of ψ the beampower can be varied. Therefore the same device can be used to achievepower control, otherwise known as channel equalisation, as well aschanging the routing or direction of a beam. Deliberate changes in thebeam shaping function can be used to increase the number of ‘greylevels’ possible for the beam attenuation, i.e. to provide an increasedresolution. As for the beam shaping, the rejected power is diffractedout of the system. Therefore this attenuation method does not increasecrosstalk.

Another technique for controlling beam power without increasingcrosstalk is to deflect the unwanted energy in a direction orthogonal tothe fibres susceptible to crosstalk.

This may be combined with yet another technique, namely distorting thebeam phase in such a way that much of the energy couples in to thehigher-order modes of the fibre, rather than the fundamental mode thatcarries the signal. The beam phase distortion may alternatively be usedalone.

In an embodiment, these methods are achieved by dividing the area of theSLM on which the beam is incident into a set of ‘power controlling’stripes. The long side of the stripes are at least substantially in theplane in which the input and output-beam are travelling. By varying therelative phase in the stripes the coupling efficiency into thefundamental mode of the output fibre is changed, and hence thethroughput efficiency of the optical system is set. This method can beapplied to a pixellated device that is also routing or otherwiseadapting a beam. In this case each ‘stripe’ would contain between oneand many of the pixels already in use.

Alternatively the long side of the power controlling stripes could be inone plane in one electrode, with the long side of the routing pixels inan orthogonal direction in the other electrode, of which either thestripe electrodes, or the pixellated electrodes, or both, aretransparent.

Alternatively the device acts solely as a beam power controller, orchannel equaliser. In this case each stripe could be a single pixel. Theset of stripes for each beam defines a block. Many blocks could beplaced side by side to form a row of blocks, with each block in the rowproviding channel equalisation for a different beam. Many rows couldalso be provided so as to provide channel equalisation for signalscoming in on different input fibres.

If a pair of confocal focusing elements is disposed between the outputfibre and SLM then the output fibre receives an image of the field atthe SLM. In this case the attenuation at the output fibre is governed bythe orthogonality between the image and the fundamental mode of thefibre. Assuming, and without loss of generality, that a perfect image isformed such that sharp phase discontinuities are preserved, it may beshown that the coupling efficiency into the fundamental mode isproportional to the square of a sum of weighted integrals. The weight isthe modulation exp jφ applied by a stripe, and the associated integralis over the area onto which that stripe is imaged. The integrand ispositive and depends on the square of the local electric fieldassociated with the fundamental mode. Each integral is represented as aphasor, with a length depending on how much of the fundamental modepower passes through the region onto which the stripe is imaged, and aphasor angle depending on the phase modulation. The net couplingefficiency is given by the magnitude of the vector summation of theindividual phasors associated with each stripe. For simple devices itmay be advantageous to use as few stripes as possible as this reducesany losses due to dead space between the stripes and reduces the controlcomplexity. With only two stripes of approximately equal area (and hencetwo phasors of approximately equal length) the possible vector sums lieon a semicircle and hence the number of possible grey levels is equal tothe number of phase levels between 0 and π, which may not be sufficient.Transverse offset of the output fibre with respect to the centre of theimage has the effect of making the two phasors unequal and hencecomplete extinction is not possible. These problems may be overcome byusing three or more stripes per hologram. For example with three stripesthe loci of vector sums lie on circles centred about the semicircletaking just two of the stripes into consideration. Hence many morevalues are possible. Increasing the number of stripes increases thenumber of grey levels and the depth of attenuation.

A fibre spool is used on the output fibre before any splices areencountered. It will clear to those skilled in the art that other modestripping devices or techniques could be used instead.

This system can also be adaptive: given knowledge of the applied phaseby each stripe and enough measurements of the coupling efficiency, thelengths of the different phasors associated with each stripe can becalculated. Given these lengths the performance can be predicted for anyother applied phases. Hence suitable algorithms can be included in theSLM or interface to train and adapt the device performance to cater fortransverse offset of the output fibre and other misalignments.

Sharp edges or phase discontinuities in this image will be eroded by theoptical modulation transfer function (MTF) but, nevertheless, where asufficient number of stripes is provided it is possible to vary thephase modulation of each and achieve a wide range of attenuation.

Ultimately what limits the depth of attenuation is the residualzero-order due to, for example, an imperfect quarter-wave plate orFresnel reflections from different surfaces inside the SLM such that thereflected light has not yet been phase-modulated. An example reflectionis from the interface between the cover glass and transparent electrode.Such residual zero orders will couple into the output fibreindependently of the phase modulation. In many cases the residual zeroorder will have a different polarisation state to the beam that has beenproperly processed by the phase modulation, so even adapting the phasemodulation will not recover the depth of attenuation.

In such cases it is advantageous to apply some routing to the outputfibre, such that the zero order is offset from the output fibre and theintended output beam is steered into a diffraction order of the routinghologram. For a many-pixellated SLM this may be achieved using thestandard routing algorithm described earlier. For a simple SLM with fewpixels, e.g. the one with the stripes in the plane of the input andoutput fibres, these stripes can be subdivided in an orthogonaldirection, that is to create a 2-D array of pixels. This howeverincreases the device complexity.

An alternative simple device is to combine it with a tip-tiltbeam-steering element, as described in Optics Letters, Vol. 19, No 15,Aug. 1, 1994 “Liquid Crystal Prisms For Tip-Tilt Adaptive Optics” G DLove et al. In this case the top ‘common electrode’ is divided into aset of top electrodes, one for each device, where each device is assumedto receive a separate beam or set of beams. Each top electrode hasdifferent voltages applied on two opposite sides. The shape of the topelectrode is such that the voltage between the electrodes variesnonlinearly in such a way as to compensate for the non-linearity of thephase vs. applied volts characteristic of the liquid crystal. Hence withall the stripe electrodes at the same voltage the device provides alinear phase ramp acting like a prism and deflecting the phase-modulatedbeam in a pre-defined direction, such that the residual zero order fallselsewhere, as required. Changing the stripe electrode voltage causesphase changes in the imaged beam but does not prevent the deflection.Small adjustments in the phase ramp can be used to compensate forcomponent misalignments and/or curvature of the SLM substrate and/orwavelength difference from the design wavelength for the tip-tiltdevice. Such small adjustments in the phase ramp can also be used toachieve fine control over the attenuation. Hence such a device would beuseful whether or not the required attenuation is sufficiently strongfor the residual zero order to become a problem. Alternatively the topelectrode can be divided into two or more areas, with the shape of eachso as to compensate for the phase vs. volts non-linearity. Varying thevoltage on the ends of each electrode can be used to offset the phasemodulation of each stripe in order to create the desired attenuation. Inthis case the aluminium electrode would be common to the device,removing dead-space effects.

In another embodiment of the tip-tilt device, the top electrode iscommon to all devices and a shaped transparent electrode is provided,e.g. by deposition, on top of the quarter-wave plate, with connectionsto the SLM circuitry to either side of the device. In this case thealuminium may act only as a mirror and not as an electrode. Again theshaped transparent electrode may be subdivided into two or more areas toprovide the attenuation. This embodiment avoids dead-space effects andalso a voltage drop across the quarter-wave plate.

In a further embodiment, such a tip-tilt device has a shaped transparentelectrode on both cover glass and quarter-wave plate. The planes oftip-tilt for the two devices may be orthogonal or parallel. With twoparallel tip-tilt electrodes the device may act as a power-controllingtwo-way switch, and also, as will be described later, can be used in amulti-channel add/drop multiplexer. With two orthogonal tip-tiltelectrodes the device can beam steer in 2-dimensions such as to correctfor positional errors. Either of the two tip-tilt electrodes can besubdivided so as to provide attenuation.

One advantageous SLM is that described in our co-pending patentapplication EP1053501.

If there is a single focusing element between the output fibre and SLMthen the field at the output fibre is the Fourier Transform of the fieldleaving the SLM. In this case three classes of phase modulation can beused to change the coupling efficiency into the output fibre. The firsttwo classes assume a many-pixellated SLM while the third class assumes afew-pixel SLM with or without tip-tilt features as described earlier. Inthe third class the tip-tilt feature may be used to compensate fortransverse positional errors in the input and output fibre.

The different classes of phase modulation result in a variable couplingefficiency at the output fibres using the following methods:

As noted above, the first class uses a many-pixellated SLM. A periodicphase modulation is applied that creates a set of closely spaceddiffraction orders at the output fibre. The spacing is comparable to thefibre mode spot size such that there is significant interference betweenthe tails of adjacent diffraction orders. The phases of thesediffraction orders are chosen such that the resulting superposition israpidly alternating in phase and therefore couples into the higher-orderfibre modes. Varying the strength, phase and position of eachdiffraction order changes the attenuation. If the long sides of thestripes used to create this alternating output field are in the plane ofthe input and output fibres, then diffraction orders landing outside thetarget optical fibre fall along a line orthogonal to the output fibrearray, and therefore do not cause crosstalk.

In the second class, again using a many-pixellated SLM, a non-periodicsmoothly varying non-linear phase modulation is applied at the SLM, inthis case the SLM acts as a diffractive lens such that the beam isdefocused and couples into higher-order modes.

In the third class, which uses a simple SLM with few pixels, the pixelsare used to apply phase distortion across the beam incident on the SLM.Such phase modulation can be considered to be equivalent to the firstclass but with a long period. The phase distortion at the SLM results inamplitude and phase distortion in the reflected beam and hence reducesthe coupling efficiency into the output fibre.

Again, all three methods require use of a mode stripper on the outputfibre. Again suitable algorithms can be included in the SLM or interfaceto train the system.

Another embodiment, not illustrated, uses a graded-index (GRIN) lenssecured to one face of an SLM, and having input and output fibresdirected on or attached to the opposite face. The SLM may provideselective attenuation, and/or may selectively route between respectiveinput fibres and selected output fibres. A requirement for stableperformance is fundamental for optical devices used in communicationsand like fields. One of the dominant manufacturing costs for suchoptical devices is device packaging. The GRIN lens architecture resultsin a compact packaged device resilient to vibrations. However, thearchitecture can have problems with spherical aberration and problems inachieving the required alignment accuracy. In particular there is oftena requirement for precise transverse positioning of the fibres. Also dueto manufacturing tolerances in the GRIN lens the focused spot in thereflected beam can be offset significantly in the longitudinal directionfrom the end face of the output fibre, resulting in an insertion losspenalty. This problem gets worse the longer the GRIN lens. Applyingselected non-linear phase modulation to the SLM may compensate forproblems such as focus errors, length errors, longitudinal positionalerrors and spherical aberration. Applying selected linear phasemodulation to the SLM and/or using tip-tilt electrodes may compensatefor problems such as transverse positional errors.

Optical systems using SLMs may individually process the channels from anensemble of channels on different wavelengths, entering the system as amultiplex of signals in a common beam. Given a continuous array ofpixels the SLM may also process noise between the channels. Hence theoptical system acts as a multiwavelength optical processor. Theprocessing may include measurement of the characteristics of the signalsand accompanying noise as well as routing, filtering and attenuation.

In a first application, the SLMs carry out attenuation, known in thiscontext as channel equalisation. A second application is a channelcontroller. A third application is an optical monitor. A fourthapplication is an optical test set. A fifth application is add/dropmultiplexing. Further applications are reconfigurable wavelengthdemultiplexers and finally modular routing nodes. In all of theseapplications the SLMs may carry out routing and/or power control and/orbeam shaping and/or sampling and/or corrective functions as describedearlier. The system to be described is not restricted to this set ofseven applications but is a general multi-wavelength system architecturefor distributing the wavelength spectrum from one or more inputs acrossan array of devices and recombining the processed spectrum onto one ormore selected outputs.

The inputs and outputs may be to and from optical networking equipmentsuch as transmission systems, transmitter line cards and receiver linecards. Alternatively the inputs may be from one or more local opticalsources used as part of a test set: either via an intermediate opticalfibre or emitting directly into the optical system. The outputs may beto one or more local photo detectors for use in testing and monitoring.Applications outside the field of communications are also possible suchas spectroscopy.

Such multi-wavelength architectures can be adaptations of opticalarchitectures used for wavelength de-multiplexing. Wavelengthdemultiplexers typically have a single input port and many output ports.These can use one or more blazed diffraction gratings: either infree-space or in integrated form such as an AWG (Arrayed WaveguideGrating). These devices are reciprocal and hence work in reverse. Henceif a signal of the appropriate wavelength is injected into the outputport it will emerge from the input port. The output port usuallyconsists of an optical waveguide or fibre with an accepting end thatreceives a focused beam from the optical system and a delivery endproviding an external connection. Now consider replacing the acceptanceend of the output waveguide/fibre with a reflective SLM: all of theprocessed signals reflected straight back will couple into the inputfibre and emerge from the input port. These signals can be separatedfrom the input signal with a circulator. Alternatively the system isadapted so that the reflected signals emerge and are collected togetherinto a different fibre.

Free-space optical systems performing wavelength de-multiplexing can usediffraction gratings made by ruling, or from a master, or madeholographically, or by etching. Usually these work in reflection butsome can work in transmission. One or two gratings can be used in thesystem. The optics used to focus the beams can be based on refractiveelements such as lenses or reflective elements such as mirrors or acombination of the two.

Referring to FIG. 12, a channel equaliser 350 has a single grating 300used with a refractive focusing element 310 and an SLM 320. To make thediagram clearer, the grating 300 is drawn as working in transmission.Other embodiments use two gratings and/or reflective focusing elementsand/or gratings that work in reflection, such as blazed gratings.

A first input beam 301 from an input port 304 contains an ensemble ofchannels at different wavelengths entering the equaliser on the sameinput port 304. As a result of the grating 300 the beam 301 is splitinto separate beams 301 a, 301 b, 301 c for each wavelength channel,each travelling in a different direction governed by the gratingequation. The grating 300 is positioned in the input focal plane of amain routing lens 310 with a reflective SLM 320 at the output focalplane of the routing lens 310. If desired, there may also be afield-flattening lens just in front of the SLM 320.

If lens 310 were an ideal lens, rays passing through the same point onthe focal plane of the lens, regardless of direction provided they areincident on the lens, emerge mutually parallel from the lens. As lens310 is not a real lens, this is no longer strictly true: howeverwell-known lens design techniques can be applied to make it true overthe required spatial window.

Hence, the beams 301 a, 301 b, 301 c that were incident upon the lens310 from the same point on the focal plane, but at different angularorientations, emerge mutually parallel from the routing lens 310, butspatially separate. Thus, the lens refracts each beam to a differenttransverse position 320 a, 320 b, 320 c on the SLM 320. At each positionthe SLM 320 displays a pixellated hologram and/or has a tip-tilt devicefor processing the relevant wavelength component of the beam. In thepreferred embodiment, the SIM 320 is a continuous pixel array ofphase-modulating elements and is polarisation independent. The width ofeach hologram or tip/tilt device compared to the spot size of theincident beam incident is sufficient to avoid clipping effects. Instead,or additionally, beam shaping may be used. The device may be controlledto deflect or attenuate the beam as described earlier, and providesoutput processed beams 302 a, 302 b, 302 c. Beams 302 a and 302 b havemoderate channel equalisation applied by a power control hologram androuting towards the output port 305 applied by a routing hologram. Asexplained previously it is advantageous to use a routing hologram as itdeflects the beams from their specular output direction and henceincreases the available depth of attenuation. Beam 302 c has strongattenuation applied in order to “block” the channel: this is achieved byselecting holograms that direct the light well away from the output port305 towards, for example, an optical absorber 306. The processed beamsare reflected back from the SLM 320 towards the main lens 310 and thenrefracted back by the main lens towards the diffraction grating 300.Assuming the SLM 320 is flat, all beams subjected to the same deflectionat the SLM 320 and entering the system in the same common input beamemerge mutually parallel from the diffraction grating. Curvature of theSLM 320 is compensated by small changes in the deflection angle achieveddue to the holograms displayed on the SLM 320. As the light beams 302 a,302 b emerge parallel from the SLM 320 they are refracted by the lens310 to beams 303 a, 303 b propagating towards a common point in thegrating 300, which (having the same grating equation across the wholearea of concern) diffracts the beams to provide a single output beam302. Note that due to the action of the lens, beam 303 a is parallel(but in the opposite direction) to beam 301 a and beam 303 b is parallel(but in the opposite direction) to beam 301 b. Therefore all beamssubjected to the same eventual output angle from the SLM 320 arecollected into the same output port 305. Hence a system may beconstructed with a single input port 304 and a single output port 305that produces independent attenuation or level equalisation for eachwavelength channel. Note that to obtain the same deflection angle forall wavelength channels, as required, the effective length of thehologram phase ramp, Ω/m, where m is the mode number of the exciteddiffraction order and Ω is the hologram period, should be adjusted inproportion to the channel wavelength. That is the wavelength dependenceof the beam deflection should be suppressed.

As described later the channel equalisation can be uniform across eachchannel so as to provide the required compensation as measured at thecentre of each channel Alternatively the channel equalisation can varyacross each channel, so as to compensate for effects such as amplifiergain tilt that become important at higher bit rates such as 40 Gb/s.Channels may be blocked as described earlier so as to apply policing toremote transmitters that renege on their access agreements or whoselasing wavelength has drifted too far. Furthermore the noise betweenselected channels may be partially or completely filtered out, asdescribed later. Hence in a second application the multiwavelengthoptical processor acts as a channel controller.

Although such processing can be applied using conventional optics themultiwavelength optical processor has a number of advantages. Comparedto a series of reconfigurable optical filters the multiwavelengthprocessor has the advantage that the channels are processed byindependent blocks of pixels. Hence reconfiguration of the processingapplied to one or more selected channels does not cause transienteffects on the other channels. Compared to a parallel opticalarchitecture that separates the channels onto individualwaveguides/fibres before delivery to a processing device (and henceavoids the transient effects) the multiwavelength optical processor hasa number of advantages. Firstly it can process the whole spectrumentering the processor (subject to the grating spectral response).Secondly the filter passband width is reconfigurable and can be as muchas the entire spectrum, reducing concatenation effects that occur whenfiltering apart sets of channels routed in the same direction. Thirdlythe filter centre frequencies are reconfigurable. Further advantages arediscussed later in this application.

By having a choice of two or more deflection angles at the SLM everyinput channel may be routed independently to one of two or more outputports. There may also be two or more input ports. It may be shown thatfor one or more parallel input beams, the action of the grating and mainrouting lens is such that all channels at the same wavelength but fromdifferent input ports are incident at the same transverse position atthe SLM. Again this is because “parallel rays converge to the samepoint”. Hence these channels at the same wavelength are incident on thesame channel processing hologram and/or tip-tilt device. As everywavelength channel is incident on a different device, the deviceresponse may be optimised for that particular wavelength. For example ifa pixellated SLM is used the deflection angle is proportional to thewavelength. Hence small adjustments in the phase ramp can be used toadjust the deflection angle to suit the wavelength to be routed. Allchannels incident on a particular transverse position on the SLM must bereflected from that same position. As this position is in the focalplane of the lens beams from said position will emerge parallel from thelens and travelling towards the grating. After the grating the beamswill be diffracted (according to their wavelength). It may be shown thatall beams entering the system in a parallel direction will emerge fromthe system in exactly the opposite direction. It may also be shown thatall beams subject to the same output angle from the SLM will emergecoincident from the system and may therefore be collected into the sameport.

Analysis of the beams at the diffraction grating in this architectureshows that the spot size required for a given wavelength channelseparation and beam clipping factor C at the hologram depends on thegrating dispersion but does not depend on the routing lens focal lengthnor the number of output ports. The beam centres must be far enoughapart to provide adequate crosstalk suppression. Hence the greater thenumber of output beams the further the beam must be steered by the SLMand lens. As an example consider just routing in 1-D, into the m′thdiffraction order with a hologram period, Ω and a routing lens of focallength f. The output beam at the diffraction grating will be offset fromits zero order reflection by a distance given approximately by f·m·k/Ω,where λ, is the optical wavelength and Ω/m is the effective length ofthe phase ramp on the hologram (as explained previously). To increasethis offset distance the length of the phase ramp can be reduced, whichtends to require smaller pixels, or the lens focal length can beincreased. In practice there is a lower limit to the pixel size set bythe dead space losses and the size of the pixel drive circuits, whileincreasing the lens focal length makes the overall system longer. Thiscan be a particular problem when there are many output ports, even whenclose-packing 2-D geometries are used for the output beams.

Referring to FIG. 14, another method is to put a demagnification stagebetween the SLM 400 and a routing lens 404. This is positioned so thatthe SLM 400 is in the object plane of the demagnification stage whilethe image plane of the demagnification stage 402 is where the SLM wouldotherwise be, that is in the focal plane of the routing lens 404. Whatappears in this image plane is a demagnified image of the SLM 400, whichtherefore acts like a virtual SLM 402 with pixels smaller than those ofthe real SLM 400 and hence a shorter effective phase ramp length. As anexample consider the two lens confocal magnification stage shown in FIG.14. In FIG. 14 f1 is the focal length of the first lens 401 and f2 isthe focal length of the second lens 403 (closer to the virtual SLM). Thedemagnification is f2/f1 while the beam-steering deflection angle ismagnified by f1/f2.

While this method for increasing the effective beam deflection angle hasbeen described and illustrated in the context of one particular routingarchitecture it could also be applied to other optical architecturesusing SLMs to process an optical beam, for routing and otherapplications. The operating principle is that the virtual SLM 402 has aneffective pixel size and hence an effective phase distribution that issmaller in spatial extent than that of the real SLM 400, by an amountequal to the demagnification ratio of the optics. The off-axisaberrations that occur in demagnification stages can be compensatedusing any of the methods described in this application or known to thoseskilled in the art.

In an alternative embodiment the input beam or input beams contain bandsof channels, each incident on their own device. In this and the previousembodiment for the channel equaliser the beam deflection or channelequalisation may vary discontinuously with wavelength.

In a third embodiment the input beam could contain one or more signalsspread almost continuously across the wavelength range. The light at aparticular wavelength will be incident over a small transverse region ofthe SLM, with, typically a Gaussian type spatial distribution of energyagainst position. The position of the peak in the spatial distributionis wavelength dependent and may be calculated from the grating and lensproperties. For such a system the beam deflection or channelequalisation varies continuously with wavelength. The pixellated SLM isdivided into blocks, each characterised by a ‘central wavelength’,defined by the wavelength whose spatial peak lands in the middle of theblock. A particular channel equalisation or beam deflection is applieduniformly across this block. Light of a wavelength with a spatial peaklanding in between the centres of two blocks will see a system responseaveraged across the two blocks. As the spatial peak moves towards thecentre of one block the system response will become closer to that ofthe central wavelength for the block. Hence a continuous wavelengthresponse is obtained. The block size is selected with respect to thespatial width of each beam in order to optimise the system response.This method is particularly attractive for increasing the wavelengthrange of a 1 to N switch.

To achieve this aim the multi-wavelength architecture described earlier,should be configured so as to allow reconfigurable routing from a singleinput port to one of a set of multiple output ports. The length of thephase ramp used to route the beam to each output port should vary slowlyacross the SLM such that the wavelength variation in the deflectionangle is minimised, or certainly reduced considerably compared to thecase for which the phase ramp length is uniform across the SLM. Hencethe transverse position of each output beam will vary considerably lesswith wavelength, with a consequent reduction in the wavelengthdependence of the coupling efficiency at the system output.Alternatively, the length of the phase ramp can be varied spatially soas to obtain some desired wavelength dependence in the couplingefficiency.

The efficiency of a blazed diffraction grating is usually different forlight polarised parallel or perpendicular to the grating fringes. In themulti-wavelength systems described above the effect of the quarter-waveplate inside the SLM is such that light initially polarised parallel tothe grating fringes before the first reflection from the blazed gratingis polarised perpendicular to the grating fringes on the secondreflection from the blazed grating. Similarly the light initiallypolarised perpendicular to the grating fringes before the firstreflection from the blazed grating is polarised parallel to the gratingfringes on the second reflection from the blazed grating. Hence, in thisarchitecture, the quarter-wave plate substantially removes thepolarisation dependence of the double pass from the blazed grating, aswell as that of the phase modulation. As is clear to those skilled inthe art, this polarisation independence requires the fast and slow axesof the integrated quarter-wave plate to have a particular orientationwith respect to the grating fringes. This required orientation is suchthat the integrated quarter-wave plate exchanges the polarisationcomponents originally parallel and perpendicular to the grating fringes.

Referring to FIG. 28 a wavelength routing and selection device 600 isshown. This device has a multiwavelength input 601 from an input port611, and provides three outputs 602, 603, 604 at output ports 612-614.

The device 600, similar to the device of FIG. 12, has a grating 620, alens 621 and an SLM 622, with the disposition of the devices being suchthat the grating 620 and SLM 622 are in respective focal planes of thelens 621. Again the grating is shown as transmissive, although areflective grating 620, such as a blazed grating, would be possible.Equally, the SLM 622 is shown as reflective and instead a transmissiveSLM 622 could be used where appropriate.

The grating 620 splits the incoming beam 601 to provide three singlewavelength emergent beams 605, 606, 607 each angularly offset by adifferent amount, and incident on the lens 621. The lens refracts thebeams so that they emerge from the lens mutually parallel as beams615,616, 617. Each of the beams 615,616,617 is incident upon arespective group of pixels 623,624,625 on the SLM 622. The groups ofpixels display respective holograms which each provide a differentdeviation from the specular direction to provide reflected beams 635,636 and 637. The beams 635, 636, 637 are incident upon the lens 621 androuted back to the grating 620.

In the embodiment shown, the beams 605 and 606 are finally routedtogether to output port 614 and the beam 607 is routed to output port612. No light is routed to port 613.

However it will be understood that by careful selection of theholograms, the light can be routed and combined as required. It would bepossible to route light of a selected frequency right out of the systemif needed so as to extinguish or “block” that wavelength channel. It isalso envisaged that holograms be provided which provide only a reducedamount of light to a given output port, the remaining light being“grounded”, and that holograms may be provided to multicast particularfrequencies into two or more output ports.

Although the number of output ports shown is three, additional outputports can be included: with appropriate lens design the insertion lossvaries weakly with the number of output ports. Although the output portsare shown in the same plane as the input it will be clear to thoseskilled in the art that a 2-D distribution of output ports is possible.

Hence the device 600 provides the functions of wavelengthdemultiplexing, routing, multiplexing, channel equalisation and channelblocking in a single subsystem or module. These operations are carriedout independently and in parallel on all channels. Reconfiguration ofone channel may be performed without significant long-term or transienteffects on other channels, as occurs in serial filter architectures.With most conventional optics (including parallel architectures)separate modules would be required for demultiplexing, routing,multiplexing and the power control functions. This adds the overheads offibre interconnection between each module, separate power supplies, anda yield that decreases with the number of modules. The device 600 has nointernal fibre connections, and a single active element requiringpower-the SLM. Each active processing operation (routing, power control,monitoring etc) requires an associated hologram pattern to be applied bythe controller but may be carried out by the same SLM, hence the yielddoes not decrease with increased functionality. Although integratedoptical circuits can be made that combine different functions, ingeneral they require a separate device inside the optical chip toperform each function. Again the power (dissipation) and the yieldworsen with increased functionality.

Further applications of the multiwavelength optical processor are as anoptical performance monitor, and as a programmable multifunction opticaltest set. In both applications the SLM may perform two or more differentbut concurrent monitoring or testing functions on two or more portionsof the wavelength spectrum. This may be achieved by applying routingholograms to the pixel block associated with said portions of thewavelength spectrum that connect optically a selected input fibre orinput optical source to a selected output fibre or output detector. Therouting hologram applied to each portion of the spectrum may bereconfigured as required in order to perform different testing ormonitoring functions on said portion of the spectrum. To each outputphoto detector or to each input optical source is applied controlcircuitry for carrying out the required tests.

Considering firstly the performance monitor, the method described laterto measure the centre wavelength of a channel may be applied to aselected channel in order to monitor the lasing wavelength. Earlier inthis application there is a description of how to measure the secondorder moments of a beam. Consider orthogonal axes u and v at the SLM.Choose the orientation of these axes such that all wavelength channelsentering the system and incident on the grating in the defined paralleldirection have the centres of their associated beams along a line ofconstant v. Hence the position along the u axis increases withwavelength. The second order moment in the v direction is related to thespot size of a monochromatic beam. The second order moment in the udirection is related to this spot size and also the wavelengthdistribution of the energy in each channel. Hence by measuring secondorder moments, as described previously, an estimate of the channelbandwidth may be obtained. The noise power between a selected pair ofchannels may be measured by routing that part of the spectrum betweenthe channels towards a photo detector. Similarly the power of a selectedchannel may be measured by routing towards a photo detector. One or morephoto detectors may be assigned to each type of measurement is allowingmany parallel tests to proceed independently on different portions ofthe spectrum. Alternatively the control circuitry associated with eachphoto detector output may be designed to be able to perform two or moreof the required monitor functions.

Hence the multiwavelength optical processor acts as an optical spectrumanalyser with integrated parallel data processing. Conventional methodsfor achieving this use either a grating that is rotated mechanically tomeasure different portions of the spectrum with a photo detector in afixed position, or a fixed grating with a linear photodiode array. Inboth cases data acquisition hardware and software and data processingare used to extract the required information from the measured spectrum.Both systems are expensive and require stabilisation against the effectsof thermal expansion. The multiwavelength optical processor has nomoving parts, can use as few as a single photodiode, and can adapt theholograms to compensate for temperature changes, ageing, aberrations asdescribed previously in this application. The multiwavelength processoralso carries out the data processing to measure centre wavelength andchannel bandwidth in the optical domain. When used in a communicationsnetwork the optical performance monitor would pass the processed datafrom the measurements to a channel controller, such as the one describedpreviously, and also to a network management system. The signal formonitoring would be tapped out from a monitor port at the channelcontroller or from a routing system or from elsewhere in the network.The monitor processing could be implemented with the same or a differentSLM to the channel controller. Monitor processing can also beimplemented with the same or different SLMs used to route beams in theadd drop routers and routing modules described later in thisapplication. The control electronics for the monitor processing can beintegrated with the control electronics for the pixel array.

With reference to FIG. 30, the programmable multifunction optical testset 900 has a multiwavelength optical processor 928 with one or moreinputs 901, 902 from optical sources, 903, 904 each with controlcircuitry 905, 906 for performing one or more tests of opticalperformance. The channel equalisation and blocking functions describedearlier may be used to adapt the spectrum of the selected source to suita particular test. The channel filtering functions described later maybe used to synthesise a comb or some other complex wavelength spectrumfrom a selected broadband optical source. A further input 907 from anoptical source 910 may be used to exchange data and control informationfrom control and communications software 929 with the same 900 or one ormore other optical test sets, allowing remote operation over the fibreunder test, or some other fibre. One or more outputs ports 911, 912 fromthe multiwavelength optical processor are connected to a set of opticalfibre transmission systems (or other devices) 913, 914 to be tested.Routing holograms are applied to the pixels associated with the selectedparts of the spectrum to direct said parts of the spectrum or said dataand control information to the selected output port. A further or thesame multiwavelength optical processor has input ports 917, 918connected to the set of optical fibre transmission systems (or otherdevices) 915, 916 under test and output ports 919, 920 connected to aset of one or more photo detectors, 921, 922 each with associatedcontrol circuitry 925, 926 for carrying out testing functions. A furtherphoto detector 924 connected to a further output port 923 is used toreceive data and control information from one or more other test sets.Routing holograms are applied to direct the signals from the selectedinput port to the required photo detector. The optical monitor functionsdescribed above can be applied to the signals. The frequency shaping ofthe source or spectrum can take place at the transmitting test set orthe receiving test set. The control electronics for the test set 927 andcontrol and communications software 929 can be integrated with thecontrol electronics for the pixel array.

Conventionally, different optical sources would be used to performdifferent types of test on the wavelength and transmission properties offibres or devices under test; a separate optical switch would be used topoll the devices under test, and an external communications link wouldbe used for communication of data and control information with a remotetest set. However, the multiwavelength optical processor may be used toprovide a multifunction programmable optical test set that is capable ofremote operation. The test set may include as few as a single source anda single photo detector and performs a wide range of tests on fibres ordevices selected from a group of fibres or devices attached to the testports of the multiwavelength processor.

A multiwavelength system with two inputs and two outputs can work as anadd/drop multiplexer. Add-drop multiplexers are usually used in ringtopologies, with the ‘main’ traffic travelling between the ring nodes,and ‘local’ traffic being added and dropped at each node. Consideringeach node, one input (main in) is for the ensemble of channels that hastravelled from the ‘previous’ routing node. The second input (add) isfor the ensemble of channels to be added into the ring network at theadd/drop node. One output (main out) is for the ensemble of channelstravelling to the ‘next’ routing node while the second output (drop) isfor the ensemble of channels to be dropped out of the ring network atthe node. If a particular incoming wavelength channel is not to be‘dropped’ at the node, then the channel-dedicated device at the SLMshould be configured to route the incoming wavelength from the maininput to the main output. However, if a particular incoming wavelengthchannel is to be dropped, then the channel-dedicated device at the SLMshould be configured to route the incoming wavelength from the maininput to the drop output. In this case the main output now has availablecapacity for an added channel at that same wavelength. Therefore thechannel-dedicated device at the SLM should also be configured to routethe incoming wavelength from the add input to the main output.

The multiwavelength optical processor described in this applicationdistributes wavelength channels across and collects the wavelengthchannels from a single SLM, allowing the SLM to provide a set of one ormore processing operations to each of the channels. However, in mostconventional reconfigurable add drop multiplexers, the routing has to becarried out in two successive stages. Usually a first 1*2 switchingstage either drops the channel or routes the channel through, while asecond 2*1 switching stage either receives the through channel from thefirst stage or receives an added channel. Fortunately, careful choice ofthe deflection angles applied by the SLM, and the sharing of the samehologram by input signals at the same wavelength, allows add droprouting to be carried out in a single stage. Hence add drop routing maybe conveniently applied in an independent and reconfigurable manner toevery wavelength channel in the multiwavelength optical processor.

An explanatory diagram is shown in FIG. 13 a.

Referring now to FIG. 13 a, an SLM 141, used in the context of themulti-wavelength architecture, has a pixel block 140 and/or tip-tiltdevice upon which a main input beam 130 is incident, at an angle m1 tothe normal 142. The main beam has a zero order or specular reflection130 a. Holograms are made available that will cause deflections at +θ₁to the specular direction and −θ₂ to the specular direction. Due to thedisplay of a first hologram on the pixel block 140, the main output isdeflected by +θ₁ from the specular direction to a main output beam 132.An add input 131 is incident at an angle a1 on the block 140, andproduces a zero order reflection 131 a. The device also has a dropoutput beam direction 133.

When the hologram applying the deflection of +θ₁ is displayed, light atthe relevant wavelength entering in the add direction 131 is not steeredinto either of the main output beam direction 132 or the drop outputbeam direction 133. Effectively it is ‘grounded’. This feature may beused to help to stop crosstalk passing between and around rings.

When the hologram applying the alternative deflection of +θ₂ is applied,the add input is routed to the main output beam direction 132 while themain input is routed to the drop output beam direction 133.

In the interests of clarity, a simplified diagram may be used to explainan add-drop using 1-D routing. This is shown in FIG. 13 b in which thepoint 134 represents the output position of the specular reflection fromthe add input while the point 135 represents the output position of thespecular reflection from the main input. When a first routing hologramis applied the main output beam is deflected by an angle of +θ₁ andtherefore the output position of the main beam is deflected by an offsetof f·θ₁, compared to the output position 135 of its specular reflection.Here f is the focal length of the routing lens. In FIG. 13 b thisdeflection is represented as a vector 136 a and the output beam isrouted to the main output 137. The beam from the add input is subject tothe same angular deflection with respect to its specular reflection andis thus deflected by a vector of equal length and the same direction 136b with no output port to receive it this beam is “grounded”. When asecond routing hologram is applied the main output beam is deflected inthe opposite direction by a vector 138 a to arrive at a drop output 139.The beam from the add input is deflected by an identical vector 138 b toarrive at the main output 137.

The example in FIG. 13 a assumes 1-D routing due to the hologram. Givenan ability to route in 2-D, either with two orthogonal tip-tiltelectrodes or a 2-D pixel array (as described previously) thearrangement of the four ports can be generalised, as shown in FIG. 15.The use of 2-D routing allows closer packing of the input and outputbeams reducing off-axis aberrations. In FIG. 15 the output positions areshown in 2-D. The point 151 represents the output position of the zeroorder (specular) reflection from the add input while the point 152represents the output position of the zero-order reflection from themain input. The hologram deflections are represented as vectors 155 a,155 b, 156 a and 156 b. Vector 155 b has the same length and directionas vector 155 a and vector 156 b has the same length and direction asvector 156 a. When a first routing hologram is applied the add inputbeam is deflected from its specular output position 151 by the vector155 b to the main output 154 while the main input is deflected from itsspecular output position 152 by the identical vector 155 a to the dropoutput 153. When the alternate routing hologram is applied the maininput is deflected from its specular output position 152 by the vector156 a to the main output 154 while the add input is again ‘grounded’ dueto deflection by the identical vector 156 b.

In this general configuration there are six variables. These are theoutput positions of the main output and drop output, the positions ofthe zero order reflections from the main input and add input, and thetwo hologram deflections. Of these six variables only three are mutuallyindependent.

For example, selection of the input position for the main input withrespect to the routing lens axis defines the output position of the zeroorder reflection, 152. If this is followed by selection of the outputpositions for the main and drop outputs with respect to the routing lensaxis then all three independent variables have been defined. Hence therequired hologram deflections are determined as is the input positionfor the add input with respect to the routing lens axis (which thendefines 151).

FIGS. 13 a, 13 b and 15 show the hologram deflections required toprovide add-drop routing: FIGS. 13 a and 13 b assume 1-D routing whileFIG. 15 assumes 2-D routing. A multiwavelength add-drop architectureusing such hologram deflections is shown in FIG. 29. Compared to othermethods for achieving add-drop functionality, the advantages are asdescribed previously for FIG. 28.

Turning now to FIG. 29, an add/drop multiplexer device 1700 has twoinput ports 1701, 1702 and two output ports 1703,1704. The first inputport 1701 is for an input beam 1711 termed “add” and the second inputport 1702 is for a second input beam 1712 termed “main in” having twofrequencies in this embodiment. The first output port 1703 is for afirst output beam 1713 termed “drop” and the second output port 1704 isfor a second output beam 1714 termed “main out”.

The input beams 1711, 1712 are incident upon a grating 1720 thatdeflects the beams according to wavelength to provide emergent beams1731, 1732 and 1733. The emergent beams 1731, 1732 and 1733 are incidentupon a lens 1722 having its focal plane at the grating 1720, and thebeams emerge from the lens respectively as beams 1741, 1742, and 1743 tobe incident upon an SLM 1722 in the other focal plane of the lens 1721.As the beams 1741, 1742 do not originate on the grating 1720 from thesame location, they are not mutually parallel when emerging from thelens 1721. The beam 1743 is from a point on the grating 1720 common tothe origin on the grating 1720 of beam 1742, and hence these beams aremutually parallel. Although the grating is drawn as transmissive and theSLM as reflective, these types are arbitrary.

The first beam 1731 and the third beam 1733 are at the same wavelength,hence they emerge parallel from the grating 1720 and are refracted bythe lens 1721 propagating as beams 1741 and 1743 respectively to a firstgroup or block of pixels 1723 on the SLM 1722. This pixel block 1723applies the required hologram pattern that routes a channel entering theadd port 1701 to the main output 1704, and also routes a channelentering the main input 1702 to the drop port 1703. Hence the firstgroup of pixels 1723 deflects the first beam 1741 to provide firstreflected beam 1751, and deflects the third beam 1743 to provide thirdreflected beam 1753.

The second beam 1732 is at a different wavelength to the first and thirdbeams 1731 and 1733 and therefore emerges at a different angle from thegrating 1720. This third beam is refracted by the lens 1721 andpropagates as beam 1742 to a second group of pixels or pixel block 1724on the SLM 1722. This second group of pixels applies the hologrampattern that routes a channel entering the main input port 1702 to themain output port 1704 and “grounds” a channel entering the add port1701. The second group of pixels 1724 deflects the second beam 1742 toprovide the second reflected beam 1752. The holograms on the first andsecond groups of pixels are selected, (examples were described for FIGS.13 a, 13 b and 15), so that the first and second reflected beams 1751,1752 are mutually parallel; the third beam 1753 is routed in a differentdirection. The consequence of this is that the first and second beams1751, 1752, after passing again through the lens 1721 become incident ata common point 1726 on the grating 1720, and emerge as main out beam1714. The third beam 1753 is incident upon a different point on thegrating 1720 and emerges into as the drop beam 1713.

In most cases ring networks are bi-directional, with separate add/dropnodes for each direction of travel. In some networks a loopback functionis required. This allows isolation of one segment of the ring in case oflink failure, for example. It also allows the transmission systems forboth directions of a link between two nodes to be tested from a singlenode. This latter function is useful to confirm that a failed link hasbeen repaired. Loop back requires the main input on each add/drop nodeto be routed to the main output on the other add/drop node, as shown inFIG. 16.

The figure shows a first module 161 a and a second module 161 b. Thefirst module 161 a has a main input 162 a, an add input 166 a, a loopback input 165 a, a main output 163 a, a drop output 167 a and a loopback output 164 a. The second module 161 b has a main input 162 b, anadd input 166 b, a loop back input 165 b, a main output 163 b, a dropoutput 167 b and a loop back output 164 b.

The node is divided into two sides: a west side 168 and an east side169. Loop back may be required for one or for both sides of the node.Channels coming from the ring enter the first module 161 a on a maininput 162 a and enter the second module 161 b on a main input 162 b. Innormal operation through channels will be routed from the main input 162a to the main output 163 a and from the main input 162 b to the mainoutput 163 b.

In loop back operation for the west side 168 the through channelsentering the input 162 a on the first module 161 a are routed to theloop back output 164 a. This output 164 a is connected to the loop backinput 165 b of the second module 161 b. In loop back operation for thewest side all channels entering the input 165 b are routed to the mainoutput 163 b of the second module 161 b.

In loop back operation for the east side 169 the through channelsentering the second module 161 b on the main input 162 b are routed tothe loop back output 164 b. This output 164 b is connected to the loopback input 165 a of the first module 161 a. In loop back operation forthe east side 169 all channels entering the input 165 a are routed tothe main output 163 a of the first module 161 a.

The function can be implemented in the four port add drop node(explained in FIGS. 13 a, 13 b, 15 and 29) by selecting a furtherhologram deflection 179 a and 179 b, as shown in FIG. 17. In the fourport architecture both sides of the node loop back at the same time.This is due to the sharing of the same hologram by input signals at thesame wavelength. In FIG. 17 the vector 179 a deflects the main inputfrom its specular output position 172 to the loop back output 176. Theidentical vector 179 b is applied by the shared hologram to the loopback input such that it is deflected from its specular output position173 by the identical vector 179 b to the main output 175. The othervectors 177 a, 177 b, 178 a and 178 b are used for normal add-dropoperation: 174 is the drop output and 171 is the specular outputposition for the add input.

When such a hologram is applied the main input is routed to the loopbackoutput and the loop back input is routed to the main output. The twoadd/drop nodes are then connected as in FIG. 16.

The loop back function can be implemented in other add droparchitectures (described later) by reserving drop ports for loop backout and add ports for loop back in. In these other architectures theloop back may be applied to just one side of the node, as well as toboth sides.

The method used to provide loop back ports may also be applied to themultiport add drop (FIG. 18). This method may be used to provide crossconnection ports to exchange channels between adjacent add drop nodes.

It is also possible to devise holograms for multicast, i.e. forwardingan incident light beam to each of several outputs. Such a hologram canbe applied to route the main input to two outputs, with vectors 177 aand 178 a (in FIG. 17). In this case the device is performing a drop andcontinue function. This is required to provide a duplicated path atnodes connecting two touching ring networks.

Alternatively, or additionally, additional inputs and outputs can beprovided so as to have a separate input for each added channel and aseparate output for each dropped channel. This saves the expense andspace taken up by additional filtering and/or wavelength multiplexingcomponents that would otherwise be used to combine all added channelsonto a common add port, and to separate all dropped channels toindividual receivers. An example layout is shown in FIG. 18. In such animplementation care must be taken that sufficient distance is providedbetween the zero order reflections from each input, and the outputpositions for each output, so as to control the crosstalk. In FIG. 18deflection v2 is used to deflect channels entering the main input fromthe specular output position m0 to the main output position m2.Deflections v4 to v7 are used to route from the four add inputs (withspecular output positions a1, a2, a3 and a4) to the main output m2.Identical deflections v4 to v7 are applied by the shared holograms todeflect the main input from its specular output position m0 to the fourdrop outputs d1 to d4. For example if wavelength channels λ5 and λ7enter on add input 2 which has its zero order (specular) reflection ata2, the holograms associated with these wavelength channels areconfigured to produce deflection v5. Hence these two channels will exitfrom the main output m2. Any channels entering the main input on thesetwo wavelengths will experience the same hologram deflection, and willthen exit from output d2.

In one implementation of the multiwavelength architecture the opticsbetween any input fibre and the corresponding input beam that arrives atthe diffraction grating, is such that the beam spot that arrives at theSLM is an image of the beam spot that leaves the input fibre. Similarlythe optics between any output beam and the corresponding output fibre issuch that the beam spot that arrives at the output fibre is an image ofthe beam spot that leaves the SLM. An example embodiment that wouldachieve this behaviour is to have an individual collimating lensassociated with and aligned to every optical fibre.

Referring to FIG. 27, it is assumed that two adjacent channels are beingrouted in a different direction to the channel under consideration. Thusthe beam under consideration has a first hologram 500, and the twoadjacent beams have contiguous holograms 501 and 502 respectively. Thebeam under consideration has an intensity distribution shown as 510.Hence the energy incident from the beam under consideration on the twoadjacent holograms, shown as 511 and 512, is lost. Given a perfectoptical system what arrives at the selected output fibre is ademagnified image of the truncated beam. Due to the way that the opticalsystem works, the centre line of the beam incident at the output fibrewill be lined up with the centre of the output fibre (indeed the beamdeflection angle at the SLM should be adjusted so this is the case).

To each wavelength channel there is assigned a block of pixels applyingthe same routing hologram. Preferably this block of pixels should bechosen such that an input light beam exactly at the centre wavelengthfor the channel arrives at the SLM such that the centre of the beam iswithin a half pixel's width of the centre of the assigned pixel block.In the presence of thermal expansion of the optomechanical assembly thecentre of said beam may arrive at a different point on the pixel blockresulting in partial loss of signal as more of the beam tails are lost.This problem can be avoided either by expensive thermally stableoptomechanics or by dynamic reassignment of pixels to the blocksassociated with each channel. For this to be achievable the pixel arrayshould be continuous. This continuity of the pixel array is advantageousfor thermal stability whether or not the imaging criterion used tocalculate the filter response is satisfied.

The way that the architecture behaves is that for all parallel beamsincident on the grating, the position at which the beam at a particularwavelength reaches the SLM is independent of the input port. Hence areference signal of known wavelength will be incident at the sameparticular point on the SLM, whether it comes in with any of the signalsto be routed, or on a separate input. The method to measure the positionof the beam centre can be used on one or a pair of such referencesignals. Given this information, an interpolation method can be used tomeasure the wavelength of some other signal entering the system on oneof its input ports, given the measurement of the position of the centreof the beam associated with said other signal. This information can beused to monitor the behaviour of the original transmitter lasers, andalso to inform the controller for the routing system.

Furthermore, given the position of said reference beams as they reachthe SLM, and also the centre wavelength(s) of (an)other signal(s)entering the system, the position of the beam(s) at said centrewavelength(s) upon the SLM may also be calculated. This information canbe used to control the adjustment of the pixel blocks and/or hologramsused to route and control said other signal(s). Conversely the positionof said reference beams may be used to select a pixel block thatprovides a given required centre wavelength for a filter. Hencereconfigurable assignment of pixel blocks may be used to tune the centrewavelength of one or more filter pass bands.

For the purpose of calculating the wavelength filtering response it isassumed that the centre of the beam at the centre wavelength of thechannel (shown as 500 in FIG. 27) arrives exactly at the centre of theassociated pixel block. With reference to FIG. 31, as the wavelength isincreased above the centre wavelength of the channel the centre line 946of the beam 940 lands at a distance 941 away from the centre 945 of thepixel block or hologram 942. As a result of the offset 941 due towavelength difference, the beam loses more energy 943 to the adjacenthologram 944. Assuming perfect imaging, what arrives at the output fibreis a demagnified image of this truncated beam.

An important difference for the multi-wavelength architecture, comparedto conventional wavelength demultiplexers, is that a wavelengthdifference from the centre of a wavelength channel does not (to firstorder) result in an offset error of the beam at the output. This isbecause of the way the second pass from the grating ‘undoes’ thedispersion of the (fixed) diffraction grating, as was shown, forexample, in FIG. 12. Hence the original centre line of the truncatedbeam should be aligned with the peak of the fundamental mode in theoutput fibre, or, equivalently, aligned with the optical axis of theoutput fibre. Standard methods for the calculation of couplingefficiency into single-mode fibres have been used to calculate thefilter characteristics. Example results are in FIGS. 19 and 20.

FIG. 19 shows the relative transmission Tlo for in-band wavelengths as afunction of the ratio of the wavelength offset u to centre of thewavelength channel separation. Each curve in the Figure is for adifferent value of the hologram clipping factor (CR) in the range 2 to4: this factor is defined as the ratio of the hologram width to the beamspot size at the hologram.

FIG. 20 shows the relative transmission Thi inside the adjacent channel,with u=1 at the centre of the adjacent channel while u−0.5 is at theboundary with the adjacent channel. Again, each curve in the Figure isfor a different value of the hologram clipping factor (CR) in the range2 to 4. FIGS. 19 and 20 also show that a change in the width of thepixel block assigned to the filter passband (that is a change in CR)will change the passband width and extinction rate at the edges of thepassband. Hence reconfigurable assignment of pixel blocks may be used totune the shape and width of the filter pass bands.

Independently of the clipping factor, the suppression at the edges ofthe wavelength channel is 6 dB and the full width half maximum (FWHM)filter bandwidth is approximately 80% of the channel separation.Comparison of the different curves in FIG. 19 shows that the flatter thefilter passband the steeper the skirts at the edges, leading to greaterextinction of the adjacent channel, as shown in FIG. 20.

This behaviour is advantageous as it avoids the usual tradeoff betweenadjacent channel extinction and centre flatness. Good centre flatnessmeans that the filters concatenate better, so more routing nodes usingsuch filters can be traversed by a signal before the signal spectrum andhence fidelity starts to deteriorate. Good adjacent channel extinctionis also important as it prevents excessive accumulation of crosstalkcorrupting the signal.

For example, in a known conventional wavelength demultiplexer the filterpass bands are Gaussian and the 1 dB and 3 dB filter bandwidths areinversely proportional to the square root of the adjacent channelextinction (in dB), such that the greater the extinction, the narrowerthe filter passband. For the same FWHM filter bandwidth of 80% aGaussian filter would have an adjacent channel extinction weaker than 20dB, leading to crosstalk problems. However for the SLM multi-wavelengtharchitecture the adjacent channel extinction is better than 30 dB,avoiding such problems in most known networks.

As is well-known to those skilled in the art, an arbitrary beam incidenton an optical fibre couples partially into the fundamental mode of thefibre with the rest of the beam energy coupling into a superposition ofthe higher order modes of the fibre. The higher order modes may bestripped out with a fibre mode stripper. The coupling efficiency intothe fundamental mode is given by the modulus squared of the ratio of anoverlap integral divided by a normalisation integral. The overlapintegrand is the product of the incident field and the fundamental mode.The normalisation integrand is the product of the fundamental mode withitself.

FIGS. 33 and 34 are included with the aim of explaining the behaviour ofthe ‘imaging filter’ as described above. FIG. 32 shows the truncatedincident beam profiles 960-964 as the wavelength is increased from thecentre of the channel under consideration, 960, to the centre of theadjacent channel, 964. Truncated beams 961, 962 and 963 are forwavelength differences of a quarter, a half and three-quarters,respectively, of the channel separation. In the diagram the truncatedbeam profiles are offset vertically for clarity. The beam profiles arealigned horizontally as they would be physically at the output fibre;the original centre of each truncated beam is aligned with the centre ofthe fibre fundamental mode. This is because, as explained above, awavelength difference from the centre of a wavelength channel does not(to first order) result in an offset error at the output. Beam 965 isthe fundamental mode of the fibre. FIG. 33 shows the overlap integrands970-974 of the truncated incident beams with the fundamental mode of thefibre, as the wavelength is increased from the centre of the channelunder consideration, 970, to the centre of the adjacent channel, 974.The normalisation integrand, 975, is also shown. The results in thefigures show that the overlap integrand 974 has almost vanishedexplaining why the adjacent channel extinction is very strong. Overlapintegrands 971 and 972 are for wavelength differences of a quarter and ahalf, respectively, of the channel separation.

* These results explain why the overlap integrand decreases slowly withwavelength difference in this range leading to a flat passband centre.In particular for the halfway case, 972, the overlap integral is exactlyhalf of the normalisation integral (from integrating 975). Hence theamplitude transmission coefficient at this wavelength difference is ahalf with a power extinction of 6 dB, as was shown in FIG. 19. Thereforetwo factors are responsible for the excellent filter characteristics.The first factor is that the field incident on the fibre is an image ofthe field reflected from the SLM. The second factor is that the secondpass from the grating undoes the dispersion applied by the first passfrom the grating, such that whatever the wavelength offset inside thecollected channel, (to first order), the peak of the reflected truncatedbeam is aligned with the peak of the fundamental mode of the fibre.

By way of comparison, FIGS. 34 and 35 illustrate the filtering processfor a conventional wavelength demultiplexer. In FIG. 34 the centre of afirst beam 984 is aligned with the optical axis 980 of the centre of afirst optical fibre or optical waveguide 981. Hence the first beam 984is at the centre wavelength of the channel collected by the firstoptical fibre 981. A second optical fibre 9B3, adjacent to the firstfibre 981, has an optical axis 982. A second beam 988 is aligned withthe optical axis 982 of this second optical fibre. Hence the second beamis at the centre wavelength of the channel collected by the secondoptical fibre, that is at the centre of the adjacent optical channel tothat collected by the first fibre. Beams 985 to 987 are at wavelengthdifferences from the first beam 984 of a quarter, a half, andthree-quarters, respectively, of the wavelength separation between thetwo adjacent channels. The coupling efficiency of each of the beams 985to 988 into the first optical fibre 981 again depends on the overlapintegral of the respective beam with the fundamental mode of the fibre981. This is mathematically identical to the overlap integral of therespective beam with the first beam 984.

FIG. 35 shows the overlap integrands 994 to 998 plotted against avertical axis 990. The spatial width and shape of each curve isidentical, as may be shown analytically. Hence the overlap integrand isproportional to the amplitude of the curve, as may be read from the axis990. Curve 994 is the overlap integrand at the centre of the channel,and is the product of the distribution 984 of FIG. 34 with itself. Thiscurve has an amplitude of 1.0 and hence maximal coupling efficiency.Curves 995 to 997 are the overlap integrands at wavelength differencesfrom the channel centre of a quarter, a half, and three-quarters,respectively, of the wavelength separation between the two adjacentchannels. Curve 998 is the overlap integrand at the centre of theadjacent wavelength channel. The coupling efficiency is given by thesquare of the amplitude of the overlap integrand. The results in FIG. 35show that the coupling efficiency for the conventional wavelengthdemultiplexer decreases more quickly around the centre of the filterpassband than for the ‘imaging’ filter discussed in this application.The results also show that the adjacent channel extinction is weaker forthe conventional demultiplexer.

FIGS. 34 and 35 also explain why there is a performance tradeoff for theconventional multiplexer between filter passband flatness and adjacentchannel extinction: to increase the width of the filter passband thebeams 9B5-986 must be incident closer to the first optical fibre 981.Necessarily the beams 987-988 will also be closer to the first opticalfibre, reducing the extinction of the adjacent channel, and requiringthe second optical fibre 983 to be moved closer to the first fibre 981.

FIGS. 32 and 33 explain why the imaging filter behaves in a differentway, such that a broader filter passband is associated with a greaterextinction of the adjacent channel. Beam 960 in FIG. 32 shows thetruncated reflected beam at the centre of the filter passband. The firstand second amplitude discontinuities 966 a, 966 b are due to the twoedges of the hologram. An increase in the hologram width relative to thespot size moves these two discontinuities outwards. The significantamplitude discontinuity in the middle beam 962 is exactly at the centreof said beam, whatever the hologram width. This is because said middlebeam is associated with a wavelength halfway between the centres ofadjacent channels. Hence the coupling efficiency for this halfway pointis 6 dB, independently of the hologram width. The significant amplitudediscontinuity in the quarterway beam, 961, is exactly halfway betweenthe first amplitude discontinuity, 966 a of the centre beam 960 and thesignificant amplitude discontinuity in the halfway beam, 962. As thefirst discontinuity 966 a moves outwards due to an increased hologramwidth (in the direction of arrow 967) the significant discontinuity inthe quarterway beam must move in the same direction, increasing theoverlap integral and improving the filter passband centre flatness.Similarly as the second discontinuity 966 b moves outwards (in thedirection of arrow 968) the significant discontinuities in thethree-quarter way beam 963 and adjacent beam 964 must move in thedirection of arrow 968, decreasing the overlap integral and improvingthe adjacent channel extinction. This explanation reinforces theargument that the two factors described above (imaging and the second‘undoing’ pass from the grating) are responsible for the excellentfilter characteristics. This explanation also explains how the selectionof the width of the block of pixels assigned to a channel may controlthe filter passband characteristics.

Analytically it can be shown that the filter response for dropping oradding an isolated channel is purely real. Hence there are no phasedistortions with this type of dropping filter. This is advantageousbecause in many ‘flat-top’ filters the phase distortions associated withthe steep skirts may distort the pulses, particularly in higher bit-ratetransmission systems for which the signal bandwidth is broader.

In these calculations it was assumed that the blocks of pixels assignedto each wavelength channel are contiguous. That is there are no ‘guardbands’ of pixels between each block. Further analysis showed thatintroducing such guard bands has the effect of decreasing the channelbandwidth for a given channel separation. Hence, preferably the pixelblocks assigned to each wavelength channel should be contiguous.Alternatively guard bands can be used to route in a third direction todeliberately narrow a channel bandwidth, if required.

While the above discussion is for the case of an isolated channel, inwhich both adjacent channels are routed in a different direction to thechannel under consideration, there are also filtering effects that canoccur when one or both adjacent channels are routed in the samedirection. These effects are caused by ‘stitching errors’ at theadjacent edges of a pair of holograms routing in the same direction. Forexample a stitching error of pi causes (in theory) complete extinctionof a light beam at a wavelength exactly halfway between the centres oftwo adjacent channels, while for an absence of stitching error at eitherside of a hologram, the transmission is uniform right across the entirechannel. Intermediate stitching errors cause intermediate extinction.This acts as an additional programmable filtering mechanism and can beused to advantage to partially or completely filter out amplifier noisebetween selected channels, if required. Alternatively when maximallyflat passbands are required the stitching error should be minimised.

As described previously, all channels entering the architecture at thesame wavelength are incident on the same hologram. This is because theinput beams are arranged to be parallel as they arrive at thediffraction grating, such that all channels at the same wavelengthemerge parallel from the diffraction grating. As the diffraction gratingis at the focal plane of the lens the beams therefore converge towardsthe same point in the other focal plane of the routing lens (orequivalent mirror) at which point the SLM is placed.

Hence for the four port and multiport add/drop devices the channelsentering on the main beam (from the main input fibre) share a hologramwith those channels at the same wavelength entering on an add port. Whenconfigured with one particular routing hologram the channel entering themain input is routed to the (selected) drop port while the channelentering the add port is routed to the main output. Therefore anychannel equalisation applied to an added channel will also beunavoidably applied to the dropped channel. Hence it is not possible tocarry out independent channel equalisation on added and droppedchannels.

This problem does not occur, however, for the devices with a singleinput and/or with a single output. This is because in these devicesthere is no sharing of individual holograms between channels entering orleaving on different ports. Nor does the problem occur for the deviceswith multiple inputs and multiple outputs, for channels routed from themain input to the main output.

Another configuration of the multi-wavelength architecture is to have asingle input port and a separate output port for every wavelengthchannel and SLM devices for each channel capable of providing a set ofmany deflections. When configured so that a single channel leaves oneach output port, the device acts as a reconfigurable demultiplexer suchthat the assignment of a particular wavelength to each output port canbe changed dynamically.

Conventional wavelength demultiplexers are not reconfigurable and aretherefore less flexible as a routing component. They also have aGaussian filtering characteristic, which is inferior to the filtercharacteristic of the SLM multiwavelength optical processor, asdescribed earlier. A further advantage of the invention, compared to aconventional free-space wavelength demultiplexer, is that the channelfilter bandwidth is independent of the physical separation between theoutput fibres and also independent of the spot size of the output fibre.In contrast, for the conventional demultiplexer, the channel bandwidthis proportional to the ratio of the output waveguide spot size to thephysical separation of the output waveguides. Consequently, and in orderto obtain sufficient channel bandwidth, microlens arrays are required toincrease the effective spot size or waveguide concentrators are used todecrease the waveguide separation.

When used in reverse the device acts as a reconfigurable multiplexer,allowing the use of, for example, tuneable lasers at each input. Incontrast, for a conventional wavelength multiplexer, fixed-tuned lasersmust be used at each input.

A system with a single input port and many output ports can act as amodule to form part of a modular routing node. If the system has Moutput ports and a single input port, then each routing device producesM different deflections, with small adjustments to compensate forwavelength differences and alignment tolerances. All devices (i.e.holograms) producing the same eventual deflection will cause theassociated wavelength channel to be routed out of the same output port.Hence such a system can send none, one or many (up to the number ofchannels entering the input port) channels out from the same outputport. The logical function of the module is to sort the incomingchannels on the input port according to their required output port, asalso illustrated in FIG. 21. Considering firstly the case of the routingarchitecture shown in FIG. 12. As there is a single input port, everywavelength channel has its own hologram. Hence independent channelequalisation may be applied for all the signals flowing through themodule.

One application of these modules is to use two of them to make anadd/drop node, as shown in FIG. 22. FIG. 22 shows a first routing module660 having one input 661 from a previous node, a through output 662 andthree drop outputs 663-5, as well as two spare outputs 666,667. A secondrouting module 670 has a first input 671 connected to the through output662 of the first module, three add inputs 672-4 and two spare inputs675,676. The second module 670 has an output 677 to the next node. Thesecond (output) module can be physically identical to the first (input)module but it is used ‘in reverse’.

The first module routes all the through traffic out on a common throughport 662 while providing multiple drop ports: one for each droppedchannel. Any single wavelength or any set of wavelengths can be sent toany drop port. Hence each of the drop ports may connect to a localoptoelectronic receiver in a local electronic switch, or to a remotecustomer requiring one or more channels for remote demultiplexing. Thereconfigurability of the wavelength assignment means that the moduleacts like a wavelength demultiplexer combined with a matrix switchingfunction, so may reduce the switching demands placed on the electronicsservicing the drop ports. The ability to send a selectable set ofwavelengths to the same port reduces the need for additionalfibre/multiplexing components and increases flexibility. Furthermore therouting applied to each wavelength channel may be multicast, as well asunicast. Hence drop and continue operation may be provided in which thesignal is routed to a drop port and also to the through port. If atransparent optical connection is required through to access anddistribution networks this multicasting may also be applied to broadcastsignals to a number of drop fibres. In this multicasting operation oneor more of the previously described power control methods may be appliedto equalise the channels on the through and drop fibres, as required forthe transmission systems and receivers to function correctly.

The first module provides any channel equalisation and monitoringrequired for the drop ports. Channel equalisation and monitoring for thethrough channels may take place in the first module, or the secondmodule, or both.

The second module provides multiple add ports: one for each addedchannel. Any single wavelength or any set of wavelengths can be receivedat any of the input ports. This allows each of the add inputs to be atuneable laser, which would not be possible with a conventionalnon-reconfigurable wavelength multiplexer. In the conventional casethere are two options for providing the added channels. A first optionis to use conventional non-reconfigurable wavelength multiplexing tocombine the added channels, because this is much more efficient in termsof insertion loss than a non-wavelength-specific multiplexer (such as a1:N fibre splitter used in reverse, that is a N:1 combiner). Howeverthis requires each input port of the wavelength multiplexer to have atransmitter laser at a fixed wavelength. When a particular wavelengthchannel is added at the node the associated transmitter is in use.However when the network reconfigures its wavelength assignment thatlaser may no longer be in use. To allow complete reconfigurability acomplete set of transmitter lasers must be provided, one for each systemwavelength. This makes reconfigurable add drop nodes uneconomic whenadding small numbers of channels, due to the large overhead of idletransmitter lasers. A second option is to use tuneable lasers, one foreach added channel. With conventional optics this requires anon-wavelength-specific multiplexer, which imposes insertion losspenalties. The multi-wavelength architecture described provides areconfigurable wavelength multiplexer with lower insertion loss than aN:1 combiner. Furthermore the routing applied to each wavelength channelcan be reconfigured without transient effects on other wavelengthchannels, as occurs in ‘serial’ multiplexing architectures that have areconfiguration capability.

Any add port can receive a reconfigurable set of wavelength channelsfrom a remote customer. The second module also provides any channelequalisation required for the added signals. Finally the second moduleroutes the through channels entering on the port 671 to the output 677.

The spare ports 666,667,675,676 can be used for routing selectedchannels to optical regenerators if the signal quality demands it; towavelength converters to avoid wavelength blocking; to another add/dropnode to allow cross-connection between rings, as shown in FIG. 23, or tofurther modules to allow expansion, as shown in FIG. 24.

FIG. 23 shows a first to fourth routing modules 720, 730, 740 and 750.The first and fourth modules each have one input 721, 751, a throughoutput 722, 752, a cross-connect output 723,753 and a number of dropoutputs 724, 754. The second and third modules 730,740 each haverespective single output 731,741, a number of add inputs 732,742 across-connect input 733,743 and a through input 734, 744. The throughoutput 722 of the first module 720 is connected to the through input 734of the second module 730, and the through output 752 of the fourthmodule 750 is connected to the through input 744 of the third module740. The cross-connect output 723 of the first module 720 is connectedto the cross-connect input 743 of the third module 740, and thecross-connect output 753 of the fourth module 750 is connected to thecross-connect input 733 of the second module 730.

The first and second modules 720, 730 are on one ring and the third andfourth 740, 750 on a second ring. This cross connection capabilityallows a new ring network to be overlaid on an original ring networkwhen the original ring capacity is becoming exhausted Channels may beexchanged between the two rings at each node as required. Hence the ringnetwork acts like a ring with two fibres per link (in each directionaround the ring). The concept may be extended to three or more overlaidrings, and hence three or more fibres per link (in each direction aroundthe ring). As is well known from many traffic studies, increasing thenumber of fibres per link reduces significantly a phenomenon known aswavelength blocking, such that more efficient use is made of thecapacity of each fibre. Hence cross connection between rings makesbetter use of the available capacity, allowing more traffic to becarried for the same investment in infrastructure. Cross connection mayalso be used to exchange signals between diverging rings.

FIG. 24 shows expansion of a first (input) module 760 having a singleinput 761, and five outputs 762-6, via an optical amplifier 768 and anintermediate module 770 having four outputs 771-4. The first output 762of the first module 760 is a through path, the third output 764 is anexpansion port and provides an input to the optical amplifier 768, andthe output 769 of the optical amplifier is to the intermediate module770. The intermediate module 770 has an expansion port 771 and three newports 772-4. Fourth and fifth outputs 765, 766 of the input module 760form drop outputs. The same principle can also be applied to expansionof a second (output) module. The use of such modules allows extra addand drop ports to be provided without service interruption to thechannels flowing through the add drop node. It also allows networkoperators to apply just in time provisioning, delaying investment ininfrastructure until the demand is there to use it. Furthermore it isonly the channels dropped or added through the expansion module(s) thatare subject to an additional amplification stage. If every node in thering were upgraded in this manner, the channels should only pass throughan additional two amplification stages. This could be reduced to oneadditional stage by suitable assignment of the added and droppedchannels to the original and expansion module.

Returning to the basic routing module shown in FIG. 21. This type ofconnectivity would be useful in mesh networks where each node isconnected by a multi-fibre link to, typically, each of between two andfive nearest neighbour nodes. Each link carries traffic to and from oneof the nearest neighbour nodes. Usually individual fibres in the linkcarry traffic in just one direction but some are bi-directional. For anexample where a link has an average of six pairs of external fibres anda node has five links, then there would be thirty external incomingfibres and thirty external outgoing fibres. The function of the node isto route any wavelength channel from any incoming fibre to any outgoingfibre. Each fibre may carry many wavelength channels. Currently up to160 channel systems are being installed although 40 or 80 channelsystems are more usual.

An ideal node architecture allows the network operator to start with oneor more add/drop nodes connected to one or more rings and then allow theindividual add/drop nodes to be connected so that the network topologycan evolve towards a mesh. The node architecture should also allow extrafibres to be added to each link as required to meet the demand, with theextra parts or modules of the node being installed as and when required.Fibre management and installation between sub-components inside therouting node is also expensive.

A known architecture for such a routing node uses a separate wavelengthdemultiplexer for every input fibre. The separated wavelength channelsare then carried over optical fibres to N*N optical switches. To avoidinternal wavelength blocking then all channels at a particularwavelength must be connected to the same N*N switch. Hence the switchwill receive channels at the same wavelength from every single inputfibre. The channels leaving the switch are carried over optical fibresto a separate wavelength multiplexer for every output fibre. Hence theswitch will route channels at the same wavelength towards every singleoutput fibre.

These switches have a sufficient number of ports for added and droppedchannels, and channels passing to and from wavelength conversion andoptical regeneration. This sufficient number is estimated based ontraffic analysis as it depends on the instantaneous mapping of channelsbetween nodes and the wavelength and fibre allocation. Each switch mayservice one or more wavelength channels. In one device, the number offibres is around b 3000 resulting in significant fibre management andinstallation costs. Even grouping together different fibres to or fromthe same link and grouping together the add fibres and regeneratorfibres only reduces the number of separate entities to be managed to560.

With such a large number of fibres it is not economic to provide opticalamplifiers inside the routing node to compensate for insertion losses.Another problem with this architecture is how to add in extra externalfibres once the switch capacity has been exhausted with the currentnumber of external fibres. This cannot be done without replacing everysingle switch. In advance it is difficult to know how large to provisionthe switch to avoid or delay this problem.

An alternative node architecture uses one of the multi-wavelengtharchitectures described to provide a separate module for every inputfibre and a separate module for every output fibre. Consider first aninput module. This should be designed so that none, one, many or all ofthe input channels may leave any of the output ports (as shown in FIG.21). These output ports are used to carry channels towards outputmodules and towards other parts of the node providing wavelengthconversion, regeneration and ports to electronic switches, for example.A connection between an input module and an output module carries everywavelength channel mapped between the corresponding input and outputfibre. Hence the logical function of an input module is to sort theincoming channels according to their destination output fibre. Thislogical functionality was illustrated in FIG. 21.

A particular input module does not have connections to every outputmodule. It does not have connections to output modules going back to thesame neighbouring node from which the input channels have travelled,except perhaps for network monitoring and management functions. It mightnot need to have separate connections to every output module for theoutput fibres to the other neighbouring nodes. It is however providedwith sufficient connectivity to the output channels on every output linkto avoid unacceptable levels of wavelength blocking. For example eachinput module could be connected to a subset of the output modules, withan overflow system used to provide a connection to the other outputmodules, when required. An output module is designed like an inputmodule but works in the opposite direction. Hence the logical functionof the output module is to collect the channels coming from each inputmodule and direct them to a common output port.

In this architecture, the dropped channels and channels needingwavelength conversion may exit from each module on a common port or apair of ports. As a result of using the modules it can be shown thatsatisfactory performance is achieved using fewer than 1000 fibres andfewer than 50 fibre groups.

Hence the total number of fibres inside the node is reduced by a factorof over 3 while the total number of fibre entities to be installed andmanaged is reduced by a factor of 10 or more. This represents asignificant reduction in cost and complexity.

An example wavelength-routing crossconnect using the modules is shown inFIG. 25. FIG. 25 shows four input routing modules 790-3, each with arespective input 790 i-793 i and four outputs 79001-79003 etc. and fouroutput routing modules 794-7 each with four inputs and a respectivesingle output 794 o-797 o to a respective output fibre. One output ofeach input module 790-3 forms a drop output. The input and outputmodules are associated together with input module 790 associated withoutput module 794, input module 791 associated with output module 795,input module 792 associated with output module 796 and input module 793associated with output module 797. The remaining three outputs of eachinput module are cross-connected to the non associated output modules,so that for example the three non-drop outputs of input module 790 arecoupled to respective inputs of output modules 795, 796 and 797.Specifically, output 79001 is connected to output module 795. Of theinputs to the four output modules, one per module is an add input andthe remainder are connected to outputs of the input modules 790-3.

In the example the routing function carried out by each input module790-3 is to sort the incoming channels with respect to the selectedoutput fibre 794 o-797 o for example, and with reference to the figure,all wavelength channels entering the cross-connect on input 790 i thatneed to leave the cross-connect on 795 o are routed by the input module790 to the output 79001. This output carries these channels to theoutput module 795 which is collecting frequency channels for output 795o. The output module combines all incoming channels onto a respectivesingle. output.

In this architecture channel equalisation may be carried outindependently for all channels routed through the cross connect.

The cross connect architecture of FIG. 25 is modular in that it can beused to build a range of nodes of different connectivity and dimension.The modules can be used to assemble a node like that described above,starting with only 1 or 2 fibre pairs per link and adding in extramodules to allow more fibres per link. Extra modules can be added in andconnected up as and when required, allowing the network operator todelay investment in infrastructure for as long as possible. When thenode has reached, for example, 6 fibre pairs per link and the capacitybegins to be exhausted there are three ways to upgrade the node. Thefirst way is to upgrade the numbers of wavelength channels on particularfibres in each link. This requires replacing the associated modules withmodules processing more channels. However the other modules (and thefibre interconnections) can remain in service. In contrast for theconventional architecture as well as upgrading the demultiplexers andmultiplexers associated with the particular fibres to be upgraded, awhole set of N*N switches must be installed, one for every new systemwavelength. These switches will remain under-utilised until all thefibre systems have been upgraded.

A second way to upgrade the node is to replace selected modules withmodels providing an increased number of fibre choices per output linkallowing more fibres per link. This requires the installation of morefibre groups inside the node. In contrast for the conventionalarchitecture every N*N switch must be replaced meaning the associatedsystem wavelengths would be out of service on every fibre entering orleaving the node.

A third way to upgrade the node is to upgrade selected modules bycascading another module from a spare, or expansion output port, asshown in FIG. 26.

FIG. 26 shows a somewhat similar arrangement to FIG. 24, and has aninput module 860, with an input 861, five outputs 862-6, an opticalamplifier 870 and an intermediate module 880 receiving the output of theoptical amplifier 870 and providing four outputs 881-4. The input modulehas three outputs 862-4 to existing output modules, fourth output 865 tothe optical amplifier 870 and fifth output as a drop output. The firstto third outputs 881-3 of the intermediate module 880 connect to new orlater output modules.

The advantage of this third way is that service interruption is notrequired during installation.

The smallest node can have as few as two modules, which would act as anadd/drop node. Several pairs of such modules can service a stacked setof rings, allowing interconnection between different rings. Adjacentrings can also be interconnected. A hybrid ring/mesh network can becreated. Hence the same modular system can be used for ring networks,mesh networks and mixes of the two. It can also allow re-use of existingplant and allow an add/drop node to grow and evolve into awavelength-routing cross-connect.

It will be clear to those skilled in the art that the use of reflectiveSLMs may allow optical folding to be accomplished and provide a compactsystem. Thus folding mirrors which may be found in some systems arereplaced by SLMs that serve the dual function of folding and performancemanagement for the system. The performance management may includemanaging direction change, focus correction, correction of non-focusaberration, power control and sampling. When taken together with thecontroller and sensors, the SLM can then act as an intelligent mirror.

As an example, this application of SLMs would be attractive in thecontext of free-space wavelength demultiplexers as it would help tosuppress the problems associated with long path lengths.

Another example is to provide correction for alignment tolerances andmanufacturing tolerances in systems requiring alignment between fibrearrays and lens arrays. In particular focal length errors in the lenses(due to chromatic aberration or manufacturing tolerance) can becompensated by focus correction at the SLM or SLMs, while transversemisalignment between a fibre and lens which leads to an error in thebeam direction after the lens, can be compensated by beam deflection atthe SLM or SLMs.

It will also be clear to those skilled in the art that although thedescribed embodiments refer to routing in the context of one-to-one, itwould also be possible to devise holograms for multicast and broadcast,i.e. one-to-many and one-to-all, if desired.

Although the invention has been described with reference to a number ofembodiments, it will be understood that the invention is not limited tothe described details. The skilled artisan will be aware that manyalternatives may be employed within the general concepts of theinvention as defined in the appended claims.

What is claimed is:
 1. An optical processor having a reflective SLM, adispersion device and a focusing device, wherein the SLM has atwo-dimensional array of controllable elements, wherein the SLM isconfigured such that each controllable element is selectable wherebytwo-dimensional groups of controllable elements are formed at chosenlocations of the SLM; wherein the processor is configured such that,using the focusing device, light from a common point on the dispersiondevice is spatially distributed by wavelength across at least one of thetwo-dimensional groups, and wherein the processor is configured suchthat each of the two-dimensional groups of controllable elementsdisplays a different hologram at the chosen locations of the SLM wheresaid light is incident, for processing said light.